Pasty butt. Warm water usually dissolves the poo. Once cleaned, dry the chick thoroughly.
Pasty butt is usually caused by stress of some sort. Is the brooder too hot? Too cool? In too noisy a place? Too crowded?
Once you get her cleaned I'd also think about some probiotics (packets from tractor supply) or some plain greek yogurt to get her gut bacteria in shape. On my last batch of chicks I added the probiotic packets to their water for the first week and only had one very mild case of pasty butt on one chick. Also remember to gently dry her off after you've removed the clump (a hair dryer on low will work)