I haven’t got a clue what this flatworm is.   At first I thought it was similar to the worm “Unknown A” on John Hoover’s site, but Cory Pittman suggested that it differs significantly from the species he photographed as this worm has dorsal tentacles while his does not.  So, once again, nothing is for certain.   Found at Makena Landing, Maui in several feet of water weaving its way all too quickly through a rock.