Feather dusters can molt their crown--could be bad, could be normal. Crowns are dropped when the worm plans on spawning, to prevent it from catching it's own larvae. But, crowns are also dropped when the worm is highly stressed. I'd keep watch on it, and if there's no sign of it coming out,, reach in and GENTLY squeeze the tube--you should be able to feel the worm inside react. If it doesn't, it's dead.
Have you been target feeding them? IME, they must be target fed to survive.