Filtration dust discovered while doing water damage restoration in a wet wall. This looks really bad, but it is very normal. The black areas on the fiberglass insulation are caused by air movement in and out of the wall. As the air moves back and forth the fiberglass traps whatever dust is in the air. Over time the filtered dust can be substantial. The more dust and the greater the time the darker the spots. It is not mold, it is mold, BOTH are true??? Yup, it is not mold in the sense that there has been no growth of mold in that spot. BUT, because there is always mold in dust, then there is mold. It also very normal and found in most buildings.