take the chick and while holding it in one hand, run warm water and use your hand to rub water on the poo until it gets soft. It will start coming off quickly and try not to pull any butt feathers while doing so. Good luck!
I had a few that had that problem. It has been really rainy here and the humidity was really high so I cleaned a vaselined behinds and added 2 more vents in my hen house and problem has subsided.