Strange substance on my coat

Went to a board game night at a brewery with my brother last night. It was in the basement of a building, and we noticed there were a couple leaks coming down from the ceiling across the table from me. When I got up to leave, I realized it was leaking behind me too, and this weird white substance was all over my coat.

What could this be?? Looks like bird shit, but nay, there was nary a bird in there.