It's a type of Limpet snail if that's what it is and they are completely safe.
Distribution: Western Central Pacific: Indonesia, Philippines and Guam
Water condition: 72-78ºF; sg 1.025; pH 8.1-8.4
Active time: Day and Night (mostly night)
Size: 3/4 inch.
Diet: Herbivorous
Zone: live rock, will clean the glass
Pros: They are herbivores and feed upon micro algae in the reef tank. They are to small to ever knock corals over. Reproduction is easy: more food = more snails
Cons: Sometime considered as a nuisance because of its rapid reproduction. Peppermint shrimp are known to eat this snail.