Got this the other night in one of my tanks. I used to have 2 lettuce nudibranches in the tank that were 2-3" long, but I haven't seen them in a long time. The other night I see this little guy on some live rock. He was about 3/4" long. Either the nudibranch shrunk, or they made babies.

Anyway, on to the pic!

nudibranch - Nudibranch picture