Why porous materials are different
EPA guidance explains that mold can grow into the spaces and crevices of porous materials, making them difficult or impossible to clean completely in some cases.
Do not just paint over it
EPA specifically advises against painting or caulking moldy surfaces before cleanup and drying. Covering growth does not address the cause.
Look for the water path
Mold on drywall near plumbing, windows, roofs or wet flooring can be a clue that moisture extends beyond the visible surface.