I can't help, but others might be able to, particularly if they have more information.
How long have you had your chickens? Did you get them all at the same time? Has this one always looked like this or is this condition new?
Is she eating, drinking, and behaving normally?
How do her beak, comb, feet, and vent look?
Are the others picking on her?
Good luck! I hope it's a simple moult or something like that.
And -- welcome to BYC!