@Djgm , how is your duck doing?
In waterfowl, bright green colored droppings often indicate excessive bile production which most frequently arises during an anorexic state. If she is currently being housed outside, it would be best to move her inside, into a quiet area, with an aimed temperature of 80F (watch for overheating exp. panting, wings help out). Birds currently in an anorexic state often have a decreased metabolic heat output and are often unable to maintain normothermia, for that reason, supplemental heat should be provided.
Anorexic birds often aren't drinking as much as they should be which can severely worsen the problem. Dripping liquids on the side of their bill is a decent start to ensuring she stays hydrated, but in the long-term is not enough. Supplemental tube feeding would be indicated for an anorexic bird to maintain nutritional status. Information on tube feeding can be found here:
https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/
If she were mine, I'd likely start off by tubing her an electrolyte such as Gatorade, or Pedialyte. After she starts producing droppings, you can start tube feeding her a warm feed mix using parrot baby feeding formula, or a gamebird feed crumble. Weighing her on a gram scale daily to ensure she is maintaining a steady weight would likely be suggested as well.
In respect to what might be the underlying cause of her symptoms, a good start at figuring out what that might be would be performing a thorough physical examination.
https://www.merckvetmanual.com/exot...ltry/physical-examination-of-backyard-poultry
If you're unable to find any clues with an exam, and a veterinarian is not an option, you can treat her on an elimination basis, and try antibiotics in consideration of a bacterial infection. Egg binding and dystocia can be common in young hens so you may consider calcium supplementation.
https://allbirdproducts.com/products/baytril-10
