Never,ever pull anything off,doing this can damage fragile skin. Soak her bottom,you can use a wide cup or plate with warm water this will soften up poop mass enough that it should come off easy. Dry her off after,you can gently towel dry or use a blow dryer on LOW setting held away from her,but close enough to dry bottom off.
After you get it off if they start pecking the area you can put Rooster booster pick no more lotion on the little one's behind and it tastes bad to them. I have to use this extensively if I get a few of them with pasty butt and am washing their behinds every day for a little while.