Initially I considered this a possible species of Sclerodoris or Rostanga, but upon reflection it does not seem to have body sculpturing or projections to fit either of those genera. Sea Slugs of Hawaii lists it as a species of Diaulula, and there has apparently been an as yet unpublished anatomical investigation, so we will put it there.
Links:
Sea
Slugs of Hawaii
Created 20 April 2009