It looks like it got pecked in the eye. You will need to clean it as gently as possible to remove the puss, then apply terramycin cream directly to the eye (it’s an ophthalmic antibiotic). You may have to separate her until it’s healed to prevent the others from pecking at it some more - once you have the ointment on there it will be shiny. Also, they are waaaaay too little to be integrated. Give them a few more weeks to catch up on size / maturity and then integrate them properly using the “see but can’t touch” method.