Mildew sits on top of a surface and wipes away. Mold grows into the material and usually means water is getting somewhere it should not. On most Grand Strand homes the dark growth is actually algae - here is how to tell which one you have.
Start with the distinction that decides what you do next. Mildew stays on top of a surface as a dry, dusty film - grey, white or light brown - and it lifts off with a cloth and a cleaner, leaving the material underneath unchanged. Mold is raised, fuzzy or slick, and it grows into whatever it is feeding on, which makes it a moisture problem rather than a dirty-surface problem. And the dark growth most Grand Strand homeowners point at is usually neither one: it is algae, the black streaking on a shingle roof and the green haze on a shaded wall. All three are handled with the right cleaning solution and enough dwell time rather than with brute pressure, and the reason to tell them apart is simple - two of them are routine coastal maintenance, and the third one is a warning.
Biological growth needs warmth, moisture and something to feed on. Along the coast of Horry County all three are available for most of the year, because the ocean keeps humidity up and there is no hard winter to reset the cycle. Surfaces here are damp at dawn more often than they are dry, and a wall that never fully dries is a wall that will eventually grow something.
Salt is the local wrinkle. Ocean-facing and near-oceanfront homes carry a fine salt film that is hygroscopic - it pulls moisture out of the air and holds it against the siding. That is why an oceanfront elevation can look dingy while the inland face of the same house looks fine, and it is also why rinsing matters as much as washing here.
The second local factor is pollen. Loblolly and longleaf pine coat everything in yellow in spring, and further inland - Carolina Forest, Conway, the older Socastee neighborhoods - live oak shade keeps the north side of a house damp all season. That settled organic dust is the food source. Growth is not a sign of a dirty homeowner; it is what happens when pollen, humidity and shade sit on a wall together.
Mildew grows on a surface, not into it. It shows up as a flat, powdery patch, often grey or faintly yellow-brown, and it favors places that stay damp without ever really getting soaked: the ceiling of a covered porch, the shaded run of vinyl on the north wall, painted trim tucked under a deep overhang, the interior face of a screened porch on a golf-course lot.
The check takes ten seconds. Wipe it with a damp cloth. Mildew smears and comes away, and what is underneath looks normal - no soft spot, no dark shadow in the grain, no chalky roughness. If it wipes off, you have a cleaning job and nothing more, and a soft wash restores the wall without touching the substrate.
Mold behaves differently. It stands up off the surface, often fuzzy and sometimes slimy, and it comes in a wider range of colors - black, dark green, orange, white. What matters is that it grows into porous material. On wood it follows the grain, and the stain stays after the surface growth is gone because the organism is already below the surface.
On a Myrtle Beach home, exterior mold usually marks where water is going that nobody intended. Look at the bottom courses of siding where rain splashes back off a walkway, the trim under a gutter seam that drips instead of draining, sills below a window whose seal has failed, and the framing under an elevated beach house where air does not move. Older painted wood on the beach cottages takes it up quickly, and so does any untreated wood near ground contact in this water table.
Cleaning removes what you can see. It does not fix the water, and if the source stays, the growth is back in weeks instead of seasons. So the order is: find the leak, then clean. Growth that returns fast in one specific spot after a proper wash is a gutter, flashing or plumbing question, and no amount of washing will answer it.
The dark streaks running down a shingle roof are not mold. They are a cyanobacteria commonly called Gloeocapsa magma, and they run downhill because they feed in part on the limestone filler in asphalt shingles and travel with rainwater. The green cast on a shaded wall, a fence, or a north-facing stretch of vinyl is likewise algae rather than fungus.
That distinction is practical. Algae is the most common growth on the coast and the most predictable: it appears where shade and moisture last, it clears with a proper soft wash, and it comes back on a rough annual cycle depending on tree cover and how much salt spray the elevation catches. It is maintenance, not damage. Roof algae specifically is treated with a no-pressure application rather than a wand - that is covered on our Myrtle Beach roof cleaning page.
Run these in order and you will usually get to the right answer from the ground.
1. Where is it? Downward streaks on a roof plane means algae. A uniform green or grey haze on a shaded wall or fence means algae or mildew. A concentrated patch at a gutter seam, a sill, or where siding meets grade means look for water first.
2. Flat or raised? Draw a fingertip across it. Flat and powdery points to mildew or algae. Raised, fuzzy or slick points to mold.
3. Does it wipe off? A damp rag lifts mildew and most algae from vinyl or painted surfaces. Mold on wood leaves a stain behind, because it is already inside the material.
4. Is the wood sound? Press it. Soft, spongy or crumbling means moisture has been present long enough that cleaning is no longer the whole job.
5. How fast does it return? Back within weeks of a real cleaning means an active water source. Back after a year or more is ordinary coastal regrowth.
The Grand Strand housing stock is mostly vinyl and fiber cement, with stucco and synthetic stucco on many of the newer coastal builds, painted wood on the older beach cottages, and brick inland. Each behaves differently.
Vinyl and fiber cement release growth easily, but they will drive water behind the courses if a wand is aimed upward - which is exactly how water ends up in a wall that had no leak before it was cleaned. Stucco and synthetic stucco hold a green film down in the texture and need dwell time rather than force; they are also the surfaces most easily damaged by an impatient operator. Painted wood on an older cottage is where mildew and early mold are confused most often, because from the ground they look identical and from a ladder they do not. Elevated homes add a shaded underside that stays damp, and that is worth a look whenever growth on the lower walls keeps coming back.
All three respond to the same approach, and none of them respond well to raw pressure. A solution matched to the surface goes on at low pressure, gets time to work, and is rinsed off. The dwell time is what kills the organism; pressure only blasts away what is already dead, and on stucco, painted wood and aging vinyl it takes some of the surface along with it.
That is why exteriors here are soft washed rather than pressure washed - the same reasoning laid out in soft washing vs pressure washing in Myrtle Beach. Flat hard surfaces like a driveway are a different case and do take a surface cleaner at real pressure. For walls and roofs, chemistry and patience do the work; see Myrtle Beach house washing for how that runs on vinyl, stucco and fiber cement.
Two coastal cautions apply to all three. Solutions strong enough to kill growth will burn foliage, so landscaping gets wet down before and rinsed after. And on any home within reach of salt spray, the rinse is not a formality - leftover salt film is what starts the next cycle early.
No house on this coast stays growth-free, and anyone promising that is selling something. What you can change is the interval. Cut back branches touching or overhanging the house so shaded faces get airflow and some sun. Keep gutters clear of pine straw so water leaves the roof instead of running down a wall. Aim downspouts away from the foundation to stop splash-back on the lowest siding. Clear the pollen and organic debris that piles up on horizontal trim and in corners, because that debris feeds the next bloom.
Most homes here settle into a full exterior wash every year or two - sooner on heavily shaded inland lots and on oceanfront elevations that collect salt, later on open, breezy lots away from the tree line. That is normal coastal upkeep, not evidence that anything is wrong.
Is the black on my roof mold? Almost certainly not. Downward-streaking black on asphalt shingles is algae, and it is treated with a no-pressure soft wash. Running a pressure wand across shingles strips granules and shortens the roof's life.
Can I just use bleach from the grocery store? It will kill surface growth, but mixed wrong or used undiluted it damages plants, corrodes metal fixtures, and can lighten paint and fabric awnings. The dilution and the rinse discipline are the difference between a clean wall and a streaked one.
Does mildew turn into mold? No. They are different organisms, not two stages of one thing. But conditions that let mildew persist are conditions mold can use, so heavy mildew that keeps returning to one spot is worth a closer look.
Is exterior mold a health risk? Growth on the outside of a house is generally a nuisance rather than an indoor air problem. What matters is what it points to - mold near a sill or joint can mean water is entering the wall, which is worth checking from inside.
Does salt air make it worse? Indirectly, yes. Salt film holds moisture against the surface, so an oceanfront wall stays damp longer than an inland one and grows algae sooner. It is one reason near-beach homes tend to want washing more often than homes out toward Conway.
How soon after washing can I paint? Let the surface dry completely first. In coastal humidity that usually means waiting well past what a paint label assumes, and painting over trapped moisture is a common cause of early peeling.
Will one cleaning fix a spot that keeps coming back? If it is algae or mildew, yes - until the normal regrowth cycle catches up. If it returns within weeks in the same place, the cause is water rather than dirt, and it needs a repair instead of another wash.
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