Okay, that sounded weird...when I said 'I've had several with it' I didn't mean several with an umbilical cord...ALL Chicks have an umbilical cord...I meant several with one that took awhile to fall off
That's her umbilical cord. I've had several with it, and it's perfectly normal...sometimes it takes a LONG time to fall off. The only thing I'd worry about is if other chicks are pecking in it. She's not got pasty butt, and there's nothing to worry about :-)