Mold on drywall is either a surface problem or a cavity problem, and almost every mistake in dealing with it comes from treating the second as the first. Paper faced gypsum is an excellent food source once it is damp, so growth on a wall is always a moisture story rather than a hygiene one. The useful questions are where the water is, how far the growth runs behind what you can see, and whether the wall has to be opened to answer either.
Surface growth, and mold in drywall behind the paint
Surface mold on drywall sits on the paint, wipes with pressure, and leaves a board that reads dry on a meter. Mold behind drywall does none of those things. The signs of mold behind drywall are a musty smell that is stronger at floor level or near an outlet, staining that reappears through new paint, a high meter reading at the base of the wall, cupped or swollen baseboards, and growth that returns in the same place weeks after cleaning. Mold behind wallpaper and mold under wallpaper are a particular case, because vinyl papers trap moisture against the board and the growth is often extensive before anything shows.
What the colours mean, from beginning mold on walls to black
People describe what they see, so the vocabulary runs black mold on wall, white mold on walls, yellow mold on wall, green mold on drywall and fuzzy mold on wall. The public health guidance is consistent on this point: indoor mould growth should be removed and the moisture that fed it corrected, whatever colour it is, and identifying the species rarely changes what has to be done. White mold on drywall and beginning mold on walls are usually early colonisation on a surface that has recently been damp; treat them as a moisture warning rather than as a different organism.
Brown mold on baseboard timber, shower walls and the usual places
Mold on baseboards, brown mold on baseboard timber and mold behind baseboard trim are the classic signature of a slow leak or a wet floor, because the bottom of a wall is where water sits and where the cavity is coldest. Mold behind shower wall linings usually means the waterproofing or the grout has failed and the cavity is being wetted every time the shower runs. In both cases cleaning the visible growth without fixing the water is a guarantee it returns, and in both cases the real extent is discovered by opening a small area rather than by argument.
How walls are tested, and how fast it happens
Moisture first: a meter across the wall at several heights, and thermal imaging to find the wet footprint. Then, if the cavity is suspect, a small inspection opening or a borescope through a drilled hole, which is how to test for mold inside walls without demolishing them. Air or surface sampling has a narrower use than people expect and mainly answers whether an area is clearably different from outdoors, not whether you have a problem. On timing, the standard guidance is that growth begins on wet materials within a day or two, which is why the drying decision is urgent and the testing decision usually is not.
Questions people ask about mold on drywall
What does mold look like on walls?
Speckling, a fuzzy bloom, or a spreading discoloured patch, in any colour from white through green and yellow to black. Colour is a poor guide to risk; area and whether the substrate is wet matter far more.
How do I test for mold inside walls?
Moisture meter readings across the wall and thermal imaging to find the wet footprint, then a small inspection opening or a borescope through a drilled hole. Sampling answers a narrower question than most people expect.
How fast does mold grow in walls?
Growth on wet building materials typically begins within 24 to 48 hours, which is why drying is the urgent decision. What follows depends on how long the material stays damp.
Is mold vs mildew on walls a real distinction?
In practice, not a useful one. Mildew is the word people use for thin surface growth on a damp finish. Both are mould, both mean moisture, and both are handled by removing the growth and correcting the water.