This is a fairly common nudibranch that seems to be rarely found due to its habit of hiding under rocks during the day.  These two animals illustrate the colour-variance in the gills and rhinophores (some orange, some yellow).  They were found in about 3 ft (1 metre) of water at low tide, in a relatively protected cove in South Maui — on a rough day when rocks were being thrown about in the surf.  For more information, please see Sea Slugs of Hawaii.