I have seen this three times - a sort of dusty debris apparently coming from my nerite snail. Very odd! I tried to get it on video but it was too far into the tank and I couldn't get a decent shot. It's like the nerite is spitting out little bits of brownish stuff, lots of it.
Yea, I think it is crap... literally. I have a whole tank of pest snails, and guess what the substrate it? Snail poo. Ok, so I got around to cleaning it yesterday and found a bunch of those nasty detritus worms I have in my 10g. Gross.