Since I have never personally had the pleasure of seeing one of these nudibranchs, I will leave the details and the more varied photos to Keoki Stender  and the Sea Slug Forum.   This particular individual was found floating in the water column, which is probably the only reason my snorkel buddy saw it. They are a fairly rare sighting and even more rare for snorkelers as they tend to be found deeper than 6 meters (20 ft) and prefer a nocturnal existence, or darker more protected rocky reefs.  To 2″ or 50 mm.