Occasionally one of my ducks has had this. In our case it was a temporary imbalance, and corrected itself. There can be different causes. I would look for a vet's help if it is chronic or worsening.
If you have a vet I would take her - Muscovies can get anemia, I think. Let me look that up.
https://www.backyardchickens.com/a/common-muscovy-health-problems
Something else might be some kind of parasite.
Also, sometimes there are - I think I read this recently - air sac ruptures.
She...