Pasty butt is when they poop and the poop, instead of squirting out (or dropping out) clings to the butt feathers. We had a baby that had this and the poop hardened across her anus and she could no longer poop...her poop hole was glued shut! Fortunately, we caught it in time, and when we gently washed the hardened poop off with warm water, all the backed up poop came out like a fountain. Before we did this, the chick was cheeping loudly, in obvious distress.
After that, we did "butt checks" every time we looked at them. Any dried on poop was immediately washed off with a wet, warm cloth.
Yours isn't poopy (or pasty) butt, but if you want to be cautious, just take a warm, wet cloth and gently wipe off the poop. Their little fluff is delicate, so don't pull, just wipe.