Green stools are usually seen with diarrhea.
Bile that is secreted in the first part of the small intestine is actually green. As stool is digested and passed through the large intestine it is turned into a darker brown color.
If stool is still green by the time it is excreted, it could mean that it went through the large intestine too fast to be changed in color. This is often called "rapid transit" or "decreased colonic transit time" and diarrhea that is green in color could be the result.
Have you visited a doctor?
Is the cause of diarrhea found?