Could be sticky eye/eye infection.
Debris, a scratch or rough mating can all cause eye infections in ducks. Their sinuses run down the back of their head, so often eye issues and respiratory issues go hand in hand in ducks. Symptoms of an eye infection include a closed eye, bubbling eye, redness or tearing. Cleansing the eye well with saline and then making sure the duck has access to a nice, deep water bowl to submerge her entire head can often clear up the problem, but if it doesn’t seem to get better in a few days, a compress of steeped chamomile tea or goldenseal can help clear up the irritation.