Observed this very light, material with a dry-snow like consistency on an up-flow Rheem HVAC unit in a hall closet. Did not appear to be insulation, all of that was pink. Home was built in '81. Anyone have experience with this material/situation?
It looks like this is at the exhaust/draft area.
If so, its probably flue gases condensing, leaving behind this powder that built up and fell down where you discovered it.