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if it is a limpit it will eat some corraline from the tank. I got one with my LR and he spends all his time on the glass eating algae. They are usually flatter than what that appears to be but it is hard to see in that pic.
I'd consider it safe until proven otherwise. If it's safe you will only see it on rocks and glass, never on any corals, clams, or other snails (except for cleaning the algae off the shell).