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!
Yup pasty butt. Instead of rinsing it down get a roll of paper towel and a bowl of watm water. Just dip some tp in the bowl and keep wiping at it. Rinse and repeat til its gone. They usually poop right after if the blockage was severe so wear gloves. My first case of this she took a VERY large and VERY smelly poop right in my hand