After googling berthella stellata, and sea slugs, it turns out there is a rather extensive forum on sea slugs out there just waiting to be used... It will take me a while to match him up. The tank is newly set up, so there really isn't a whole lot in there yet. He came with the rock, from TBS (Florida, Atlantic, Gulf slug/hare). I saw something of the same color swimming at night fast and very erratic, so fast I couldn't keep a flashlight on it long enough to allow my eyes to focus (weeks ago). After reading about some of the swimming sea hares, makes me wonder if it was this guy swimming around. He's locked up in the external fuge for now.