The individual documented here was so tiny (3-5 mm) that it was barely visible to the naked eye.  It fit into its background so well as to be almost invisible (and indeed was to fellow snorkelers).  This little one was found on a ball of algae that was about fist-sized.  Its cerata were completely intact until I put my glove up to its foot to see if it was actually a sea slug or an extension of the algae  Within seconds it began to autotomize its central cerata (a defense mechanism) revealing a green-flecked notum and palm-like (some even tree-like) digestive glands in its lateral cerata. (see progression in photos below). I have only seen two other sacoglossa similar to this (the Polybranchia jensenae & the Polybranchia samanthae) and so turned to Cory Pittman for help with id.  His comments and description of this animal are noted on the Sea Slugs of Hawaii site.