Many people ask about a snail or slug with a half-shell on its back. They are called Stomatella, and are fast moving slugs. Their shell is similar to a fingernail in shape, which covers the head and torso of the creature. They are found moving about the rockwork especially at night, and are an excellent part of the clean-up crew. They do breed in our tanks and you may see smaller ones, but I've never seen them reach plague proportions.
These grazers are Reef-safe.