The other part of all this is feeding.
Is it a big Hawaiian feather duster? Are you feeding large amounts of DTs or other phyto?
Besides getting picked on it may just be starving to death -- sure they will regrow smaller crowns, but that is also an attempt to grow a crown that is "tuned" for smaller particle size food.
I would aggressively try to feed it different kinds of filter food, and include spot feeding behind the crown.
The tiny feather dusters are easy to keep in the aquarium, but the larger ones require a lot more food -- you can tell how happy they are by how "nervous" they are -- if there are no predators bothering it and it's well fed, sudden noises don't generally tend to scare them (at least that was the case with my Hawaiian large duster).
If he was not very well fed, he was very nervous and skittish. And in my 6gal, I could literally turn the water green with DTs phyto and he would clear it all in about in an hour.