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...
Stomatella have much more body exposed and a smooth rounded shell, this looks more like a limpet. Limpets are typically reefsafe but I have had one that ate coral, the coral eater was ~1” long though