You could take her to a vet for a culture or get some Terramycin eye ointment at the feed store and treat her eye with it. Something has either irritated her eye or she has an infection starting . I'd get it under control asap. She doesn't run around with her flock?
@chickenman486 does your duck have a bucket of water to wash out her eyes and clean her bill? You could try adding some ACV to her water 1-2 tablespoons to 1 gal of water it can kill some bacteria but if this has been going on for sometime and she has been using a bucket to clean then you...