I agree with the others. It is called "pasty butt" and is very common in new chicks- especially shipped chicks.
I test the water with my wrist under the faucet, then hold the chick's backside under the gently running water for a bit. Then remove, wait a few seconds and repeat until it is softened enough to come off without yanking all the fuzz out.
Then I apply Rooster Booster Pick No More Lotion on the vent area so that the chicks don't peck at the newly washed bum. This must be done every day until the pasty butt stops recurring...it can take days and days!
Keep your brooder temps as recommended as cool brooders also can lead to pasty butt- and also if they are too hot and get dehydrated.
McMurray Hatchery's FAQ page for Chick Care has the temp recommendations if you need them. (Or just ask.)