Came across this today - it is in central S.C. on the lake, home was built in 2001. There was definitely moisture in the crawlspace, standing water on top of the vapor barrier and significant condensation coming off the ductwork. There was ventilation present.
My first guess was insect poop, and it was all over the floor joists. After looking at it very closely, it almost looked like a small spore that had a short stem attaching it to the joist. I also came across a section with this orange stuff that I’ve never seen before. The orange stuff definitely looks like some sort of growth to me but I don’t want to sound the alarm on the entire crawlspace if it’s just poop.