I am hoping for some knowledgeable answers... one of my hens had a bare spot on her “butt”. Today I notice her butt feathers are “dirty” looking and look loose and hanging down... mites? If so, what is the quickest way to rid her of them? She seems fine otherwise. Eating, drinking, laying...
Here is a good article with pictures to help with identifying lice/mites, and treatment options. http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html
If it is not mites or lice then it's likely from runny droppings getting in the feathers and caking there. In that case I'd take some fresh droppings into a vet that will do a fecal float test for you to check for internal parasites, which can cause runny droppings and dirty butts.
I would also give her a butt bath in warm soapy water, and rub off the droppings after soaking for a bit. Trim any excess fluff, dry off, and get a clearer picture if anything looks abnormal. Poopy butts can irritate the skin, attract flies and maggot, and are usually a sign of diarrhea.