Hi there... let me see if I can help you out.
If your horse has a sudden onset of green watery poop there is termed acute colitis. For some reason her intestinal tract is either not drawing any water out of the ingesta or is secreting excess water into her intestines. This can be caused by a number of things:
1. GI upset from something she ate: mustard seeds or weeds in the hay
will cause liquid diarrhea
2. Disturbance of the normal GI flora due to sudden change in diet. If you have recently changed to a richer hay or if your horse was on pasture but is no on hay or grain these kinds of changes can cause upsets in thenormal bacteria that live in the GI tract and cause diarrhea.
3. Parasites: If your horse hasnt been dewormed recently there coudl have been a sudden reproductive "burst" in intestinal parasites that weakened the intestinal wall and caused a problem.
4. Bacterial infection leading to colitis: Diarrhea can be caused by infections from Salmonella, Clostridium, E.Coli etc.
If it is an infectious cause usually the horse will feel quite depressed and dumpy, colicky, and will have a fever (temp > 101.5 F).
In any case the main concern with diarrhea is keeping the horse hydrated...even if they are drinking water, they can not keep up with the loss if the diarrhea is voluminous. If your horse has a fever, feels dumpy, is acting colicky, or won't eat or drink for you then I would recommend having a vet out as soon as possible.
Things you can try at home...a dose of banamine paste, bland diet (grass hay in small feedings), bio-sponge paste, deworming (if hasnt been done recently), probiotics to replenish normal intestinal flora.
I hope this has been helpful and I hope your horse feels better soon.