She may be beginning to molt if you are seeing new feathers coming in on the neck.
You want to make sure you air out the coop really well if you bleach. I suggest that you wear a respirator for your protection as well, you don't need to be breathing in all that chicken dust or the fumes from cleaning (disturbing any mold too).
Something you may want to look into would be using Oxine
https://www.revivalanimal.com/product/oxine-animal-health-ah?sku=15103-208
Use with the activator for cleaning. If using on the chickens themselves, use without the activator.
If you have a shop vac, those are great for coop cleaning/getting up excess dust. I of course, try to dust out my coop, but use a vac as well to clean up too.