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You can soak the babies butt in warm water. Don't pick it off as you can hurt her. Once it is soft and washes off, get her dry with a hair dryer.
Probiotics or Apple Cider Vinegar in the water will help with pasty butt. Human probiotics work well. Empty a capsule into a quart waterer and fill with water. Discard the capsule. Change this and make new daily. For the Apple Cider Vinegar, 3/4 teaspoon ACV in a plastic only quart waterer and fill with water. Make a new batch daily.
Pasty butt can be caused from dehydration, (especially on shipped chicks) chilling, over heating and stress.
Watch them closely for a few days and soak the butts as needed. The probiotics or the ACV will help to clear it up.