The lighter version of the Black-margined Nudibranch documented here was actually photographed in Raja Ampat.   The Hawaiian animal is significantly darker than those generally seen in Australia/Indonesia.  Both the images of the small group feeding on brown sponge and the lone Black-margined were found in less than 10 ft or 3 metres of water off of South Maui.  Sea Slugs of Hawaii provides some great images and text describing their feeding  tenacity (when challenged by polychaete worms).  These are a relatively large dorid, reaching up to 90 mm, or just shy of 4″.