It's called pasty butt and it is pretty common in chicks and keets that have been shipped. But it can also happen with chicks that you have hatched. One way I believe to prevent it is to get them hydrated by drinking before their first meal. But that isn't going to help you now.
It's important to get their butts cleaned off now. If too much builds up they won't be able to poop and they could die within a day or so. The good news is, pasty butt usually only happens the first week or two.