Tia,
Last weekend I noticed that one of my roos also had a "clumpy dirty behind". Of course this is the only roo that will peck my arms when I hold him. Well I held him and took him to a wash tub with warm water and a bunch of paper towels. Held them against his butt and let it soak in. Slowly, and I mean slowly, it started to loosen up enough for me to remove all of the stuff.
I escaped with only a few scratches from his feet because I had jeans with a hole in the leg and a few pecks and pinches on my arms. Nothing too bad.
I don't know if this was the most effective or best way to clean him up but it worked. He is all nice and clean now.