I have the same problem with one of my Rhode Island reds too. I don't have an answer for you though, I just wash her bottom from time to time. Hope someone does have some good feedback on this topic because I wish I knew what else can be done.
I just pull it off along with any feathers quick and easy, just yank them out and then there will be no more problems in the future. sure they give a little squak, but doing all that washing and stuff takes forever and makes more of a mess than you might figure. I prefer less feathers in the rear area for health and sanitary reasons not to mention it helps with fertility. But I am sure those with named pet chickens would prefer the whole giving it a warm scented bubble bath and a cup of tea afterwards treatment LOl it's all good whichever way you decide is best. keep her vent area treated with a salve or bag balm to prevent this from happening again and make sure she is drinking, dehydration is the main cause for cluster butt.
Thanks everyone!!!!!! am new at this and at this forum. Great help! what is salve or bag balm to treat her with ???? is that medicated? Is it like vaseline? ... and the poop spot is larger than you think! will it bleed if I pull it off? it looks like it is stuck to skin.