The liquid from her nares and the swelling in her cheek are typical symptoms of a sinus infection. It could also be an abscess but I think if so, it is caused by an issue in her sinuses.
Do your ducks have clean water deep enough to dunk their whole heads in? That's very important so that they can flush their nares out and to some extent prevent this sort of issue. It can still happen of course, but its one thing to consider and fix if not.
For treatment, she really needs to be seen by a vet, who would be able to confirm diagnosis and offer treatment (especially the appropriate antibiotics, which are important).
If a vet is not an option for you, then we will discuss next steps for home treatments. You will need to give her a course of antibiotics, and fresh clean water for her to dunk her head in while you treat her. Personally I would separate her from the flock and monitor her more closely, just incase it could be a respiratory illness, though I find that very unlikely. It would be easier to treat her if she is isolated and close by to monitor her breathing. I wonder if VetRX could be useful here to help her sinuses to clear?
@Miss Lydia what do you think?