Of course it's name is Gary, lol. It looks like a pond or bladder snail, a general term for pest snails in that shape. Mine are darker brown...except when small. Watch out for jelly-like egg masses on anything; glass, substrate, decor, plants, filter tubes. They are almost clear & may be quite small. You can scrape them off w/a fingernail & vacuum them out but there will probably be more. You might want to remove (or squish) Gary or you may have a lot. It's time to be tough & observant. Feed very lightly, the more food, the more snails.