This is a very small gymnodoris, generally reaching no more than 1/2″ or 12 mm.  At first I thought this was a juvenile Gymnodoris okinawae due to it obvious white markings, but the club-like rhinophores were clearly different in shape.  Though they are apparently common, I have found only two of them and both were at the base of batches of algae in approximately 12 ft or 4 metres of water.  Please see Sea Slugs of Hawaii for more photos and taxonomy.