She may have something in it or a chicken may have pecked her or it could be the start of an infection, first off does she have water deep enough to dunk her head into wash her face? place 1-2 Tab of Apple Cider Vinegar into a gallon of water make this her only means of water so she dunk s her head into to wash her eyes out. ACV is good at killing off bacteria and also helps with healing and is a good all round tonic so it's safe for the whole flock to drink and use. You can also try rinsing her eye out with a saline wash and if you have some or can get it, [TSC] Veterycin spray for wounds and infections is good to use and have on hand in the med kit. Try using the ACV first or saline wash.One of our runner ducks, Daisy, just developed an eye problem. She keeps her left eye shut most of the time and when she opens it it looks milky. Not sure if this is an injury or some kind of infection? We currently have two runner ducks, who just started laying a few weeks ago and are very productive, in a mixed flock with about ten chickens. Here's a picture with the eye open:
