Could you post a picture so we can see if there is anything you can do.
What i found that works great is putting the chicks butt under warm running water and gently rubbing it off.
Than blow dry the chick so he doesn't get cold. Keep him warm!
The cause of pasty butt is fluctuating temperatures when the brooder is too warm or too cold. Happens alot
I hope this helps
How to post a picture
Where you are typing there are 12 boxes on the top of your typing box.
The 12 boxes say Size A B IU
Then there are 2 attachment boxes. Then finally the picture box looks like a mountain with the sun on the left side. You click on that button and click on upload files. Then you choose the picture you want from your computer or phone. And upload them to BYC it is that simple
If this doesn't work or you don't understand just ask.