It's definitely from the Chromodorididae family like
Chromodoris aspera, but it doesn't have the yellow/orange rim or dotting around the edge of the skirt and the spots on the body of
C. aspera are typically more uniform in size.
That said, I'm thinking it's either a
Chromodoris sp. (
C. aspera being the closest I know from this genus) or a
Hypselodoris sp. (
H. bertschi being the closest one I know from this genus). If they weren't only reportedly found in Hawaii, I'd guess
Hypselodoris bertschi, but - either way, Chromodoris or Hypselodoris - it's most likely a sponge eater that needs incredibly specific sponges to feed on (so it'll probably starve in your tank, OP):
