I don't have ducks, but if she were mine, I'd keep the eyes flushed/cleaned with saline, then put a dot of Terramycin eye ointment in the eyes a few times a day.
Yes to the saline and Terramycin (if you can get it - I think they made it prescription only now)
What is their water situation ?
Ducklings need to have 24 hour access clean water that’s deep enough to dunk their whole head into the water and keep their eyes and “nostrils” (nares) cleared out.
A lot of the conventional waterers that are circular just aren’t deep enough for the ducks to get their eyes into the water to keep them clean and rinsed so they keep trying to get their little heads under the water and this is why so much water splashes out. I make these for my ducks
I start with holes that are small so they can not get their whole body in there just their head and then gradually upgrade to larger holes
But perhaps your waterers are fine and maybe another duckling has pecked this one in the eye and just irritated it , maybe accidentally, or perhaps she’s being picked on so keep an eye on that you may want to put a separation screen into the brooder temporarily
How is she otherwise eating and drinking and poopingand running around OK?