That is pasty butt, and it can be fatal. It's dried poo that sticks to the down and will block the vent, preventing the chicks from getting any more feces out. It must be cleaned off and cleaned of quickly! Take her to the sink and using warm water and paper towels, get as much off as you can. You may have to pick at with a toothpick as well your fingertips to break it up. You will really have to work at it to get it all off, but don't leave any there. Yes, it's uncomfortable for the chick, but not nearly as miserable as having a bottom full of poop that can't get out. After she's cleaned off, you can use a hairdryer to dry her off before she goes back into the brooder. It's not necessary if you were able to blot of most of the moisture from the down - I did it the first few times but after that I just made sure most of the water was blotted and skipped the hairdryer. But before you do put her back in with the others, coat the area around the vent with a little olive or coconut oil - Vaseline works too but just a little bit. You need to try to keep more poop from sticking to the same area again. You may also have to treat her for pasty butt more than once. Keep an eye on her. And watch the other chicks as well - if one has it more than likely others might too.
Some say it's from keeping the brooder too warm. Some say it's from temps that fluctuate too much. But I don't think anyone knows for absolute sure why some chicks get it and others don't even when they are in the same brooder. All I do know for sure is that chicks raised under a broody hen rarely -if ever - get it, and personally I think that might be because chicks raised under a heat lamp eat 24/7, unlike under a broody where they tuck under her when the sun goes down and don't eat again until morning, a much more natural cycle for them. Good luck with it!
Welcome to BYC! Hope that information helped.