Found this today, any ideas???

Found where the plumbing entered the wall under a 2nd floor vanity in a centre condo townhouse unit. Have seen plenty of moulds, (molds), and insulations over the years but this looked strangely different from anything I’ve seen in the past. What are your thoughts? Have you seen this before?
I gave the client plenty of warning and said I would ask further advice.




Looks like a build-up of dust from air infiltration/exfiltration over years and years. Especially evident in the third picture.

I agree with Brian.

Ditto…looks like dirt to me.

Dirt/dust. Is there a ducting distribution system in the house? Maybe the vapour barrier is not sealed if it is on an exterior wall? Any other mechanical fans?