If it is a mineral deficiency and it's not affecting the others, then it could be that particular bird's specific inability to process of synthesize nutrients. Personally I think it may be a disease or inherited disorder.
Why is the neck that way? It seems the bird is hunching its head into its shoulders or something. I don't know, it just looks strange. And the upside down neck feathers are also strange. Does the chick have anything else different about it?