This semi-transparent flatworm is quite beautiful in its fine lace-like detail — but I’m not sure what it is.  It appears to be a mystery and is fairly rare.   My identification relies on Cory Pittman’s documentation of a similar worm on John Hoover’s Flatworm site.  This polyclad was found in under 2 metres of water in North Kihei, was approximately 50 mm when stretched out and it moved very quickly.