You got yourself a cowrie. Depends on the species you may want to move it out. Some of them are usually reef save as it will eat algae like any other algae eating snails. Some of them will get quite distructive when they get bigger and post a threat to your inverts and corals. They are very beautiful to watch because unlike other snails that grow all kinds of stuff on their shell, cowries have a layer of membrain over its back thus nothing grows on its shell and it have the smoothest, shiniest shell among all mollusks.