Stool may turn green due to bile pigment in the stool. Fast-moving poop like diarrhea doesn’t allow the intestinal chemicals and bacteria to break down the bile pigment to its normal color. It also can be from eating a diet high in leafy vegetables or something as simple as green food coloring. 

If you notice green poop that doesn't seem to be due to what you eat, and continues, it's best to check with your doctor just in case.