I concur. Limpet. Good to have. They are known to eat hydroids which is an added benefit if you have hydroids. If you don't - no biggie, they'll scrounge like any snail.
Hydroids are little pests - sort of similar to aiptasia, but they almost look like a tiny cluster of zoanthids. Frequently they find their way into a system via live brine shrimp. They have several life stages but they are usually seen in little clusters like this: