Well, I've left the lights off for a few days, and it seems to have quelled the algae problem a little bit. The algae on my snails hasn't gone away, but now I've noticed that there's a feather duster attached to one of the snail's shell! It's got the little tube and the radiole, and if you get near the glass, or get near the snail's shell with the forceps, it darts back into the tube.