I had the same thing happen to a couple of my chicks, some had it some didn't.
I took a small bowl of warm water, an old soft toothbrush and dipped the the toothbrush in the water and then to the poopy butt. Once you get it soft, you can pulled it off with a paper towel.
After I got them cleaned up, once I put some Olive oil around the area to keep it from building up. The second time they got it, I put some vaseline on the area.
I think someone on the board also suggested mixing their starter food with some yogurt too.
I haven't had any other ones get pasty butt since I did this.