I really would not leave it with mama. She too will peck it. If there is blood or a sore of any type, chickens will peck it. Once they taste blood, that little chicken will not survive. Chickens are meat eaters too. I had a mama peck her little baby's foot just because it had a black spot on it. Eventually, he lost that toe. That is why we named him Tripod.Aw, poor baby.
I would leave it with mama and just see what happens. @Clap p 1 had a dorking chick that was just like that, it was a brooder baby though. You might want to read their thread about sweet little Seven. I wish you luck in whatever decision you make. If you decide to cull there's nothing wrong with that, but you could definitely give her a chance. Mama will probably cull on her own if she thinks it's bad enough.