This appeared to be some kind of runny poop at first, but Uncle Flo's a sucker for punishment and took a good, long whiff.  There was hardly a smell at all, and it certainly wasn't poop.  It was directly under his big 1500g male chondro, though.  Leading theory:  a bit of poop was missed during a previous spot clean, and mold grew.

Weird, but interestingly cool!