Pasty butt is usually the result of dehydration and temps in the brooder being too high. Even though you have supplied a water source this chick may not be keeping hydrated.
I find that most chicks reared in a brooder, as apposed to under a hen , need to be shown the water. Holding the chicks bottom under warm running water and loosening the poop is better than pulling at dried poop as it will do tissue damage. Prep H may be a little harsh so it might be worthwhile to dab a little witchazel ( astringent ) on the vent and then cover with Vaseline to prevent the poop from sticking . The fact that the area is enlarged also makes it a target for siblings to peck at , so be mindful that you may at some stage need to isolate this chick.
