This is a smallish flatworm (25 mm or 1 inch) that somewhat resembles cf. Prosthiostomid 3 at first glance, but it is not quite as elongated and it is noticeably thicker and fleshier.  Acotylean A has fairly conspicuous, but small, nuchal tentacles just in front of its dark brown midline stripe.  This stripe can be completely bordered in white or partially so.  May be endemic to Hawaii.  As far as we know its documented sightings have been limited thus far to Maui and Oahu.  Please see John Hoover’s flatworm site for more information and photo