Stomatella, they look like a cross breed between a slug and snail.
They are great additions to the reef tank, and feed upon algae of about everything in your system.
They will reproduce in your tank as the climb if possible to a high point in your tank and release the sperm and eggs, this will give you more stomatellas and your corals will feed in their turn on the sperm.
Cause they don't have great shell protection hermit crabs will go after them, and when still small fish will make a meal out of them too.