Does anyone know what this is? My best guesses are either some type of mold or something insect related. This is on drywall in a finished basement connected to an exterior wall. I live in Chicago, in case it matters. This wall is also adjacent to, and the "thing" is just below, a venting duct cased in drywall. It also seems that this hapoens at either corner of where the drywall duct intersects with this wall. The back yard water spigot is also in this vicinity of the exterior wall. I bought this frame house with vinyl siding last year and shortly after had mold remediated in this room. A few months later we noticed this on the wall. Knocking off the puffy black substance and treating with bleach seemed to take care of it until now. It's back! Yuck! It seems to appear at this quickly and at this stage. I need an idea of what it is so I'll know who to call; a mold specialist, exterminator or some other specialist.