First, let's just point out that green poop isn't a cause for concern as long as your baby is content, eating well, growing well, and there isn't blood in her stool. Still, you're right that any poop changes do happen for a reason. (You might not always find the reason, but there has to be one.)
Green, frothy stools can be a result of baby receiving more foremilk than hindmilk. (The first milk that comes out of your breast is lower in fat and higher in lactose than the milk at the end.) This increase in lactose moves through baby's system very quickly and can result in green poop. Sometimes this happens when mom has an especially forceful letdown, or has an oversupply of milk. If baby is otherwise, healthy, happy, and gaining weight, nothing needs to be done. If baby is having trouble eating or isn’t gaining properly, a lactation specialist can help you find a solution (like expressing a little milk before baby latches on, or feeding a few times from the same side to decrease your milk supply).
Green watery stools with a foul odor can be a sign of diarrhea, especially if it's super-frequent. Baby diarrhea can be caused by a virus, infection, stress or a food intolerance.
Green mucousy stool is a sign that baby's intestines are irritated. If this lasts a few days and then starts getting better, it's probably a virus or a mild reaction to something in mom's diet. Teething can also be the culprit: When baby swallows a ton of drool, it can irritate the intestines and cause some mucus in the poop.