Pasty butt, as it's known on BYC, happens sometimes, I think when the chick gets stressed in some way.
Keep checking the chicks and keep those vents clear and clean. A chick can die from a blocked vent.
Make sure you have the right temperature for them, good food, and plenty of fresh water. Adding chick vitamins to the water may help if this little one had a difficult hatch. With good care and a little time, the pasty butt condition should stop. It can take a week or so. Keep an eye out.