I just had this problem with one of my ducks a few days ago. First three days I tried what you did (kept her inside, washed out her eye several times, removed the white stuff with a cottonswab which I first cleaned with boiled water) without any improvement. Luckily I found a vet who was willing to prescribe an ointment. This was the first time ever a vet wanted to do something for my ducks so I was really happy... After two days ( today) she is completely herself again #blessed!
Good luck with your Duck! Do you know if she still can see trough that eye?