Not to worry, it’s your chick’s umbilical cord! Be sure not to confuse this with pasting, which is a poopy butt, this does need immediate attention. Usually, the umbilical cord falls off immediately during hatching, but sometimes it can hang around for a few days (or it falls off but the belly button takes a while to heal, leaving a scabby spot). This can happen whether you hatch at home or order from a hatchery.
It’ll eventually fall off on its own, and the spot will heal as it does for a human infant. Don’t try and remove it. If the other chicks seem to be picking at it, you may have to separate this chick until it does fall off, but that isn’t usually necessary.
TBN Ranch Chicken Keeping Resources 