Common names: Ducksbill Limpet, Elephant Slug, and Shield Slug

These critters are not slugs at all, but a snail with a shield-like shell. The shell is normally covered by flaps, but the flaps are sometimes withdrawn exposing part of the shell (as seen in the picture on the middle of it's back). They belong to the family Fissurellidae, which includes limpets.

The general consensus is these are algae grazers, however they could eat coralline algae, too. Monitor for improper behavior.