Pseudoceros concinnus

The excellent Singaporean marine life ID site put us right regarding Pseudoceros concinnus. Pseudoceros monostichos is a different marine flatworm, that is not endemic to the Koh Phangan area.
It looks like the yellow striped beastie is actually P. tristriatus (with 2 very feint extra stripes just visible) but we’re not quite sure yet.