If all the water parameters are in check, usually a Feather Duster (family Sabellidae) Hawaiian = Sabela penicillus,
Will drop its crown (radiole) when water quality declines abruptly, being tormented/harassed by a more aggressive species of fish or invertebrate or to reproduce. The Hawaiian species is notorious for releasing its crown if becoming somewhat stressed. Give it time; it will grow another within a few weeks. I guess that target feeding is just that much more important with the emergence of a new radiole.
Good Luck