Wash that butt off under running water. You need to make sure there aren’t any mites or lice or nits on her. There is a reason they get poopy butt...you just need to figure out what is causing it. It is also helpful if you have a fingernail brush nearby to comb through the butt feathers while they are being washed. I have found that this helps clean the feathers off really quickly. I just hold the chicken with her head in my elbow and stick her butt under some warm running water. Good luck!