These beautiful nudibranchs are quite tiny, relatively long and thin.  As a size  comparison, I’ve provide an image of a 2 inch Imperial nudibranch making its way over top of a Violet-gilled’s gills.  They get to be 20 mm or 3/4 of an inch — so look closely at those tiny tubular shells and bits of elongate fingered sea weed.  As a fellow snorkeler commented upon first being shown one: “oh you mean that tiny stick?”    They fall into that category (for the novice) of  hypselodoris look-alikes and are often initially mistaken for the Locust or Pease’s nudibranch — and even the Danielle’s or Kahuna. Their deep red/magenta and white rhinophores and violet and magenta gills separate them from the rest.   Keoki Stender has documented the Island Nudibranch, another potential look-a-like, but it is limited to the Northwest Island chain, is at least twice as big and lacks the magenta markings.  See  Cory Pittman and Pauline Fiene’s Sea Slug Forum site  for more information.   Endemic to Hawaii.