The ones we actually see here
Cladosporium. Probably the most common indoor mold in this region and a frequent finding in Rockville basements. It appears as olive-green to brown or black growth with a suede-like texture, and it tolerates cooler conditions than most, so it thrives on damp block walls, around window frames where condensation collects, on painted surfaces, and on fabrics and stored materials in humid spaces. It is a well-known allergen. A great deal of what people call black mold is actually Cladosporium.
Penicillium. Blue-green with a powdery, velvety surface. It spreads quickly on water-damaged materials, so it is a standard finding after any water event: wet drywall, carpet pad, insulation, and wallpaper. It also grows readily on food and on damp fabrics. Common allergen, and it moves fast once material stays wet.
Aspergillus. A large genus with many species, appearing in white, yellow, green, brown, or black. It shows up in HVAC systems, on damp walls, in crawl spaces, and in dust. Some species are allergenic; some can cause infection in people with compromised immune systems, which is why immunocompromised households warrant extra caution.
Stachybotrys chartarum. The famous one. Dark greenish-black, often with a slimy or wet appearance rather than a fuzzy one. It requires sustained saturation — days of it — on cellulose materials such as paper-faced drywall, cardboard, wood, or ceiling tile. In Rockville homes it turns up after real water events: basements that flooded, wall cavities fed by a supply-line leak, or areas under a failed sump. It is less common than its reputation suggests.
Alternaria. Dark green to brown, with a wooly or downy texture. Frequently found in showers, under sinks, around window frames, and anywhere condensation is chronic. A common allergen, particularly for people with existing sensitivities.
Chaetomium. Cotton-like when young, darkening with age, and often accompanied by a distinctly musty odor. Associated with chronic water damage on drywall and wallpaper, and its presence usually indicates a long-running problem rather than a recent one.
Aureobasidium. Pink, brown, or black, commonly found on caulk, grout, painted surfaces, and behind wallpaper. If you have persistent staining on bathroom caulk, this is a frequent culprit.
Why the species usually does not change the plan
Homeowners often want the identification first, on the theory that a benign species means they can relax. In practice the response to visible growth on porous material is the same regardless of what the lab says: contain the area, remove the affected porous material, clean and dry the structure, and correct the moisture source. Nobody leaves colonized drywall in a wall because the species turned out to be Cladosporium.
Identification is genuinely useful in four situations: documenting a health concern with a physician, supporting an insurance claim, supporting a landlord or tenant dispute, and post-remediation clearance testing where you are comparing before and after. Outside those, the money is better spent finding the water.
What color does and does not tell you
Very little, unfortunately. Multiple species produce black growth, several produce green, and the same species can look different on different substrates and at different stages of growth. Texture is slightly more informative than color — slimy versus powdery versus fuzzy narrows the field — but visual identification by a homeowner, or by a contractor standing in your basement, is not reliable. Only laboratory analysis identifies species.
What is not mold
- Efflorescence. White or grey crystalline deposits on block or concrete, left as water moves through masonry and evaporates. It is hard and gritty, crumbles between your fingers, and often dissolves in water. Not mold — but proof that water is passing through the wall.
- Mildew. Colloquially a flat surface growth on tile, grout, and fabric. It is a form of mold, generally the easiest to clean, and typically indicates a ventilation problem rather than a structural moisture problem.
- Soot and dust staining. Ghosting patterns on walls and ceilings from candles or from air movement through insulation gaps, sometimes mistaken for growth.
When to call someone
Call regardless of species if the affected area exceeds roughly ten square feet, if the growth is on porous material like drywall or insulation, if it followed a flooding or sewage event, if there is a musty smell you cannot locate, or if anyone in the home is asthmatic or immunocompromised. Our inspection and testing page covers what an assessment involves, or call (301) 231-1938 and we will match you with a licensed local pro.