how new is this tank?
has the tank cycled?
My feather duster prefers slow steady flow. After talking with Dr. Shimek about them he suggesting moving it to a place where the crown can remain fully open without swaying in the current.
these guys filter constantly, and need a near constant supply of plankters, so in a new tank I'd be quite concerned that it's not getting any food. Target feeding doesn't work very well I'm told, they have no way to store the food, in the wild the feed 24/7.
I suggest you go to reefcentral.com and search the "Ask Dr. Ron" forum on feather duster or saebellid worm. That will help you quite a bit.
If the feather duster looses its crown, DON'T PANIC, and leave the tube alone. in a week or two it may re-emerge . I had a tank crash and mine lost it's crown, I thought for sure he was dead, 6 days later I noticed something in the tube...