ID for Strange Worm on Glass

[Beverly]

Have seen this 3" worm on the glass of my 42 gal reef a few times during the day. Was so freaked by its head that all I could do was stare at it Today had the presence of mind to take a pic

Doesn't look like a bristleworm because of the "mop" head, and BWs don't usually come out in broad daylight. Photo taken at 4:30, just around feeding time. WTF is this thing?!!??