This is a beautiful little flatworm with a dark velvety centre, marked with a wide orange and narrow black margin.  This particular one was found on top of a pinkish ascidian (feeding?) perched above a hole in the rock that it quickly disappeared into.  Approximately 1″, this glorious flatworm was found in a shallow lagoon in South Maui.  There are several look-a-likes, that have garnered this species a cf. designation (as compared to).  For more information and photos, see John Hoover’s Flatworm site.  I’ve found a similar species, Pseudobiceros 13, a couple of times in North Kihei, but its raised blueish central medial line, white spots and markings set it apart.