I usually get a few chicks with pasty butt. You do have to clean that off because then that dried poo will block them up, and they can die from that. Pasty butt is usually a sign that they are too chilly. Now, if they are all staying clear of your brooder light, then raise it up just a bit.
I think after about the third or fourth day, those few chicks will outgrow the pasty butt, but just check them a few times a day until you don't see it anymore.
You can clean it off with some warm water and a paper towel--you've kinda gotta rub it between the paper towel and your fingers and crush it off, and if it's really stuck sometimes you can just trim those rear feathers just a tiny bit and that will solve your problem.
That pasty poo gets dry and hard pretty quickly, and can be really difficult to clean off with a paper towel and be careful not to pull it off, you can tear that delicate skin. What I've found works best, if you are in the house where it's warm, is to put warm water in a bowl and hold them in the warm water while gently cleaning the poo away with your fingers. They may struggle a little at first, but relax pretty quickly, almost seem to like it. Then put them under the heat lamp. You don't usually have to do this for very long, they grow out of this within a few days in my experience.