This is a creamish/whitish flatworm with brown splotches that continue nearly to the edge of its body. There is no noticeable margin but it has very obvious marginal tentacles.  As it traveled across the rocks at a fabulous speed it took on an oval, round and elongated shape. There is still some debate as to what to call this species, so I will refer you to that discussion on John Hoover’s Hawaiian Flatworms site.  Approximately 1.75 in” or 45 mm.  Found at Makena Landing, S. Maui in less than 1 metre of water in a surge zone.