Look at this beauty. Wow, striking. Nice shot Mike. The only issue I have is who is it? It actually has external anatomical features of several known species. Too bad we don't have any internal information.

When I compare Mike's photo here with those in NSSI 2nd edition, I see similarities to Nembrotha chamberlaini, N. megalocera,Nembrotha's sp. 2 and sp. 4, the latter being the closest, if you take variation into account.

To me it's the gill color, the patch on the dorsum and the rings around the rhinophores and base of gills, that are characteristic.

Until we have a specimen or two I guess we will just have to go with "I don't know."