This bright white, snow-coloured firmly-mantled nudibranch definitely stands out on the reef.  The only individual I have ever seen was in about 10 ft of water, snuggled into a small cave ledge and difficult to photograph.  .  See Sea Slugs of Hawaii if you are wanting more images and confirmation as to whether it is a Scott Johnson’s nudibranch (i.e, it has a cream coloured margin and softer body, is also white with blackish gills and rhinophores).  To about 2″ or 5omm