It looks like the umbilicus to me. Nothing to worry about. As far as a supplement, I swear by Avia Charge 2000. It's kelp-based and full of all sorts of vitamins and minerals that are excellent for chicks/chickens. I use that in every waterer for chicks, and then when they reach about 12 weeks, I use it in about every 3rd waterer. Try some natural probiotics, such as plain, all-natural yogurt. It's fun watching them try to figure out if it's a treat or something weird to step in!