Black Mold vs. Mildew: How To Tell What's On Your Wall

Every dark spot on a bathroom wall gets called 'black mold,' but mildew and mold behave very differently — and only one of them is a surface problem.

Mildew: the surface dweller

Mildew is flat, powdery or slightly fuzzy, and usually gray or white before it darkens. It lives on surfaces — tile grout, window sills, shower caulk — and wipes away with household cleaner. If it comes off completely and the surface underneath looks intact, you likely had mildew.

Mold: the tenant with roots

Mold looks raised or splotchy, comes in green, black, or brown, and often shows up in patches that grow back after cleaning. On porous materials — drywall, wood, ceiling tile — its root structure penetrates the material itself. That's why bleach 'works' for a week and then the spot returns: you cleaned the visible growth and left the organism.

The real tell: moisture

Mildew happens where humidity condenses. Mold happens where something is feeding it water — a slow leak, a poorly sealed window, a wet wall cavity. If growth keeps returning in the same spot, stop cleaning it and find the water. That spot is a symptom.

When growth covers more than a few square feet, keeps returning, or shows up after a known leak, bring in a licensed remediator. Anything smaller: fix the moisture first, then clean.