Stomatella perhaps.
But seriously, there's no way to provide an ID for a blurry grey blob. At minimum pleaee include a detailed description along with observed behavior.
Oops, I should have looked at the pic before posting. This one might be a little better, but it was with a camera phone. It resembles a snail with no shell and it moves like a snail just faster. It just moved on the back side of my tank where I can't get a pic.
