Any chance of getting a lighter picture? It almost looks like a fleshy limpet to me (the white foot?) but that is really just a guess. I really can't tell from the photo. Sorry.
Well he is still hiding so I havent been able to get a better pic. but I seen your post on another thread about seaslugs and checked that out. And there it was a Scutus sp. also known as elephant slug, shield slug, or ducksbill limpet. Cool little booger.