Great question — answering as the UphillTech team:
That definitely looks like mold growth, likely caused by a past or ongoing moisture issue—possibly from a refrigerator leak or condensation buildup over time. Even though the drywall feels dry now, mold can remain dormant and reactivate with future moisture exposure. Simply priming and painting over it won’t solve the issue; you should at least clean it thoroughly with a mold-killing solution (like hydrogen peroxide or vinegar) and ensure the area is completely dry. If the mold has penetrated the drywall, replacement may be necessary. Given the hole in the flooring with a plastic tube, it’s worth investigating whether it's related to a water supply line or drainage. Have you checked behind...
Full answer by John W: https://diy.stackexchange.com/a/315936 (CC BY-SA 4.0).
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