retention of the sheath like that is affecting one of my birds too, a 5 year old roo, who developed it after a serious infection of some sort last summer. I haven't yet found an explanation for it. A particular virus causes this (amongst other things) in Antipodean parrots, but that's probably just coincidence.
If your bird has had this since first feathering, and made it to 14 weeks, whatever caused it is probably not going to kill them, and your bird may well have acquired resistance to the disease or infection. If the other birds don't pick on it, I would put it back with them; they are social creatures and get depressed if isolated.
It would normally grow several sets of juvenile feathers while growing up; has that been the case, and the retained sheaths keep coming? Just in that area? Maybe the problem will sort itself out at a proper moult.
I helped my roo by brushing him with a little pin brush (designed for guinea pigs) and a soft bristle shoe brush, which worked better than anything I else I tried to break the keratin coating and release the feathers. A few remain on his tail, as we'd both had enough by then and he doesn't seem bothered by them. I posted a photo of what came off through that brushing on another thread, here
https://www.backyardchickens.com/th...rescued-chickens-thread.1502267/post-27253795
btw, welcome to BYC
@Jenny7