Ya, thats a black limpit. It has a shell, but it's body comes up and over to cover it. Scott just had one in the soft coral tank and gave it to themissinglink because it eats coral. Now we know why our mushrooms have been disappearing! It does eat cyano and other algae so it's good, but not for a reef tank. If you have Julian Sprung's Invertebrates book, it's on page 113.