I love this nudibranch!  Between February 2019 and January 2023 I have come across three animals, all at varying stages of development and all solitary individuals.  The first sighting was of a juvenile (approximately 40 mm).  The second was around 60 mm (just over 2″) and the third was fully mature — at least 150mm or 4″.  All were photographed in rocky boulder areas, in dead or dying coral and all were found in less than 2 metres of water.   They are a flat, stiff animal and can, at first, appear to be a flatworm with rhinophores and gills!   Apparently, they can exhibit a trailing behaviour (documented on Bill Rudham’s Sea Slug Forum).  and by Rebecca Bicker on the Sea Slugs of Hawaii site.  So keep an eye open for another individual in the area and a pale orange swirl of eggs.   Apparently, the 40 mm individual bears the colour of a juvenile. They can range in colour and some are quite dark.  For more images and information, see Keoki Stender’s site (blueish one there) and  Sea Slugs of Hawaii.