This is apparently a relatively common flatworm, perhaps because it is easily found by tide-poolers and shallow snorkelers.  It is fairly round and can reach up to 70 mm or 3″.  Both individuals were found at Makena Landing in approximately 6 ft or 2 metres of water in an area of surge and tossed, algae and sponge covered rocks.  For more information check out John Hoover’s  Hawaiian Flatworms page.