Not positive, but I think they are fresh water limpets. I had a breakout of them with some new plants a few months ago. I would clean them off my class and do a waterchange, but they would be back in a couple days. I could not get rid of them, untill I got a couple assasin snails, and they wiped out the limpets.
Only problem I have is I have tons of assasin snails now, lol