This Thordisa is easily mistaken for a Thordisa albomacula — at least that was the case for me.  Both nudibranchs have tenticular papillae that are slightly varied in height, but the setosa’s tenticular processes tend to be a bit longer and it lacks the tell-tale white patch/band that extends directly forward of the albomacula’s branchia, lending a trumpet-like appearance to their branchial gills.  The setosa is described as generally’darker, lacking in the granular ridges typical of the albomacula andthe papillae sometimes have brown tips.  There are two animals documented here — the lighter and smaller  one has its tenticular processes withdrawn.  For more taxonomic info please see Sea Slugs of Hawaii