This is a very rare sighting in shallow water and our group spotted two separate individuals on two separate days in less than 1 metre (3 ft o water). It was in an area of abundant blue octocoral, one of its food sources.  Look for a teensy (18 mm max) white spot on a rock  and you may be lucky enough to see it move.  Found both in algae and under a rock inside and outside of the fish pond in North Kihei.  See Sea Slugs of Hawai for more information and photos.