I agree pasty butt, as @007Sean said check the temp in the brooder. Make sure they can get away from the heat. Maybe so Sav-a-chick will help too. I would also make sure you wash all off their butts. So sorry you have lost one.
They are with a mom, they are about 4 weeks old and there are over 20 of them with one single silkie mom, we live in south Florida so temperatures are in the 80s. They are kept outside.
I would wash them, keep a eye on them for continued pasty butt. that many chicks, you might want to go ahead and bring them in, make sure to watch the others as well.