Feather duster worms are known to drop their crowns and leave the mucous membrane tube they live in. what we commonly see is not the actual worm but the home it builds for itself. my guess is that your worm has abandoned its tube and lost its crown. with proper water parameters i have seen them regrow both. but it should be in the sand for this. Google and see if you can find any examples of this.
Have you seen your feather duster since this started?
Have you seen your feather duster since this started?