Usually green stool is an indication that you are getting a lot of chlorofyl in your system from green vegetables, and shouldn't be a concern. But, (and pardon my being so graphic) does your stool float, too?
This is a sign of giardia, which you can pick up from your dog, cat, handling shoes that were in a watery area outside, just about anywhere. If this persists, and/or if the stool is green and floats, you will need to see a doctor for a diagnosis and a pill that will knock it out. If you don't take care of it, then you and your animal (presuming that you have one) are reinfecting each other, your animal should also be checked by a vet. Watch for diarrhia in your pet.
Otherwise, it is usually what you are eating.
Checked this out some more, and here are some other causes of green stool, I'd get checked out just to be safe:
Other causes of green stool include:
•Medication side effects
•Irritable bowel syndrome
•Infectious diarrhea - especially salmonella and giardia