This is a teensy nudibranch, reaching no more than 16 mm and it usually fits in with its surroundings quite well, making it almost impossible to see.  The one that I photographed out on a white rock mid-day is showing unusual behaviour, perhaps because the intertidal zone where it was was full of rolling boulders and carved out rocks.  As of December 2021, WORMS list this species as Phidiana semidecora, hence the two names listed in page title.   For more information and photos, please see Sea Slugs of Hawaii.