Green is the commonest color mold comes in, which is exactly why it tells you so little. Several of the genera found in houses are green at some stage, the same genus can look different on wood than on plaster, and the color changes as a colony ages. What decides what to do is not the shade but the area, the material and where the water is coming from.
What is green mold, and why the color names nothing
Green covers a large part of what grows indoors, and identifying a genus by eye is not something the guidance supports. The CDC puts it plainly: no matter what type of mold is present, you need to remove it, and sampling cannot tell you whether it will make a particular person ill. A laboratory can name what is on a swab, and you still fix the same leak afterwards.
Green mold on wood is reading the moisture, not the timber
Wood holds water and releases it slowly, so growth on a joist, a subfloor or the back of a skirting is a sign the material has been damp for a while rather than wet once. Surface growth on sound timber cleans off. Where it has gone into the grain, or the wood is soft under a screwdriver, the question has moved from cleaning to whether the timber is still doing its job, and that is a repair rather than a wipe.
Green mold in house rooms: where it turns up and why there
It follows the water and the still air: the back of a wardrobe against an outside wall, behind furniture on a cold wall, under a sink, in a bathroom with an extractor that vents into a loft, around a window with failed glazing. The pattern on the wall is the useful evidence. A tide line says liquid water arrived, an even bloom across a cold surface says condensation, and the two need different fixes.
Green mold in basement corners, and what causes green mold to keep coming back
A basement is cool, often under-ventilated, and frequently below the water outside it, so it is the first place growth returns. The cause is nearly always one of a handful of things: ground water through the wall or floor, a plumbing leak, condensation on cold surfaces in humid air, or drainage outside that puts water against the building. Treating the surface without changing one of those buys a few months.
Green mold on drywall, and the line where cleaning stops
Paper-faced drywall is food. Growth on the painted face of a sound sheet can be cleaned; growth that has come through from the cavity, or on a sheet that was saturated, is a cut-out rather than a clean, because the paper and any wet insulation behind it stay contaminated. The CDC's line on soaked porous material is the practical one: what cannot be dried is replaced rather than cleaned.
What to do about it, in order
Find and stop the water. Dry the materials and prove they are dry rather than assume it. Clean what is cleanable and remove what is not, containing the dust so you are not moving the problem to another room. Then fix what let the water in, because that is the only step that decides whether you do this again next year. Where the area is large or the material is structural, that is the point to bring in a firm.
Questions people ask about green mold on wood
Is green mold dangerous?
The CDC's position is that whatever type is present, it needs to be removed. Reactions vary between people, allergy and irritation are the common ones, and people with asthma or weakened immunity are more affected. None of that changes with the shade, which is why no authority publishes a safe color.
What is the difference in black mold vs green mold?
Color is not a species and neither name identifies one. The response is the same: stop the water, dry the materials, clean or remove. The only practical differences come from the area involved and whether the material is porous, not from which of the two you think you are looking at.
Can I paint over it?
No. Paint over living growth fails, hides the spread and makes the next inspection harder. Clean or remove, dry, fix the water, and paint last if you want to.
Does green mold mean the wood has to come out?
Not by itself. Surface growth on timber that is still hard and sound cleans off. Softness, staining deep into the grain or movement under load are what move it into a repair, and that is a judgement made with something sharp and a bright light rather than by the color.
Do I need a test to know what it is?
Rarely. The CDC's answer is that you cannot rely on sampling to know whether someone will become ill, that good sampling is expensive, and that there are no set standards for an acceptable quantity in a home, so the work does not change with the result. A test earns its money when a dispute or a claim needs a document, not to decide whether to clean.
It came back in the same corner. Why?
Because the water is still arriving. Same corner, same season, same result: look for a cold surface in humid air, a leak above it, or drainage outside putting water against that wall. The growth is the symptom and the corner is telling you where to look.