I'm guessing pasty butt. Chicks we've had have gotten it and survived, but you will need to remove the waste material. Many on here suggest gently washing it off with warm water in a sink and then drying the chick off with a hair dryer on a very low heat/no heat.
I think some chicks get it that are stressed from being shipped via mail from a hatchery or your brooder might be too warm. Try Googling "pasty butt" or searching for it on BYC. There will be lots of suggestions for helping your chick. Good luck!