This beautiful worm was found in a lagoon at Hekili Pt, Maui in less than 2 metres of water (4 ft).  At first I thought it was a Pseudocerotid J, but it lacks the tell-tale patch at the back.  It was also quite a bit bigger (40 mm) than any Pseudocerotid J I have seen.  It was spotted on the side of a dead coral/boulder in the middle of the sand in broad daylight, but quickly burrowed into a hole in the sand.