Welcome to the confusing and colourful Lilac Dorid, also called the Discodoris lilacina, the Discodoris fragilis, Tayuva lilacina and Doris lilacina.  There are three different individuals photographed here — a very tiny (.3 in or 8 mm) bluish juvenile (close to a blue food sponge), a small, more lilac coloured one (1 in or 2.5 cm?)  and a larger brown one (about 2 in or 40 mm).  One of them looks rather tattered — but apparently, they have this strange ability to literally shed their mantle at the slightest disturbance. As per usual, check out these links to more info:  Sea Slugs of Hawaii and the  Sea Slug Forum