Well if it's actually a nudibranch, it's not algae grazer. All are strictly predatory.
There are a couple of common suspects. Dendrodoris nigra is a nudibranch that feeds on sponges. It's one of the few that actually does well in the hobby, which means it probably feeds on common hitchhiking species.
Scutus is a black shield limpet with a white shell that's hidden by a fleshy mantle. Sometimes the shell can be seen through a slit in the mantle. This guy is a grazer that's mostly harmless but may occasionally taste sessile inverts.
Stomatella is a grazing snail with a reduced, flat shell. They're normally gray, but can be jet black too.
