Doesn't look like a skin irritation, it look slick she is getting picked on by the hens, or rooster mounting. How much space do you have? Often hens pick on each other because of boredom, or she is just at eh bottom of the pecking order. Is she usually picked, on or down on the pecking order? Can you post pictures of you rest up to show us the possible reason this might be happening. Good luck, Avery
To help things grow back, you could spray vetrycin on the bald area and maybe get her a chicken saddle. You can also make you own, but just a warning, my chickens got there wings cut up because of the ones with the wing strap. You may wan to buy the ones with the neck strap, and then get some extra elastic to make a strap the goes around the belly to keep it in place. I am working on my own design like that for my hens, but can get the neck strap the right size.
Or you could skip the whole chicken saddle issue, and get Pick No More or Blue Kote from a feed store, which will keep them from picking. Pick No More tastes bad so they peck it and stop because of the taste, and Blue Kote is a dark purple so it covers up the wounded area to draw there attention away from the bald back and skin. Sorry for the LONG reply, Good luck with your hen, Avery
Coop is 6x20 and run is 30'x40' covered with bird netting 14 chickens. I have some bluecote I'll spray today. Don't see any signs of aggressive pecking/fighting in the flock, then again who knows what their doing when not looking.