This is probably one of the most beautiful sea slugs I have ever seen.  Unfortunately, it was slightly damaged, so a photo of a completely unblemished individual was impossible.  Still . . . the colours and the excitement of finding  it was probably up there with finding my first nudibranch.  This Elysia is definitely not common and quite difficult to see when it blends in with the slightly iridescent bryozoa I found hiding in.  For further photos and information see Sea Slugs of Hawaii