Pyramid snails very bad!
However if its big enough check the top of the shell... if it has a hole it could be a limpet...
Limpets are great in the tanks and i had none for 2 years, then they exploded in population. keep an eye on em they get to a nice size and mine bury themselves in the sand causing it to mix up.
Limpets come in all types too so check yours out and look for more, mine happen to start out like rice and then get a black shell with a hole in the top, thus a keyhole limpett