Given it is relatively large compared to many of the dendrodoris in Hawaii (to 113 mm or almost 3″), this nudibranch seems to be quite elusive. The only animal I have ever seen was found by a friend in the tossing surf and sand of Ma’alaea Bay. It’s not in great condition and we transferred it to a more protected area (hence the gloves).  The name “elongata” suits it well as it is indeed a long narrow species — easily recognized by its white star-like patches and tubercles on its light beige-brown mantle.  The younger ones have a more white/cream mantle with brown spots and it would take some expertise to determine they were the same species.  Please see Sea Slugs of Hawaii for better images and some taxonomic info.