Hi there!
The pecking order of a flock is subject to change at any time. All that needs to happen is for one hen to challenge another, and it's on! Your girl is definitely moulting - there is no doubt about that. The missing feathers on her rump don't look like moult though - I think that's definitely feather pecking. As @WthrLady
suggested, if you can get hold of some Bluecote or No Peck, use that for sure. I have heard mixed reviews about the pinless peepers, some people swear by them and others are convinced they don't work. But for a few dollars, why wouldn't you give them a go? It certainly can't hurt to try them! I'd buy a set for all the birds you see pecking at her and see if it helps.
Another thing to think about - pecking can have several causes. Lack of protein is the main one of them, and generally speaking the birds who need the most protein are the ones who are moulting, or have recently been through a moult. I would suggest introducing some more protein into their diets for a few weeks and see if that helps. Mine like bacon, sandwich meat, and left over meat from our dinners. If you can get mealworms, they would be great too. Protein is the main thing needed to regrow lost feathers. Some birds not only peck out their flock-mate's feathers, they also eat them in an attempt to replenish their protein stores! So yes, boosting their protein intake might help you.
The other culprit with feather pecking is boredom. If left to their own devices, chickens will find a way to keep themselves entertained - read: Chase down a flock-mate and pluck out all her feathers for fun! To prevent this sort of behaviour, you need to keep them occupied. You could try dumping some logs or tree branches in their run to hide in. Freeze some fruit into ice-blocks for them to play with and eat. Install a chicken swing. Hang heads of cabbage so they have to jump to get to them. You know - a chicken play-gym!
Hope some of that helps you. If your girl is at the point where she is bleeding from her attacks, I'd separate her out. If chickens see blood it can spell the end for the unfortunate wounded bird - they just can't leave it alone. Otherwise, if there is no blood drawn, I'd push on with protein and entertainment and see if that helps.
Bets wishes,
Krista