Is it formed or loose? If you have diarrhea it could just indicate a rapid transit time.
Have you been eating anything with artificial color...green or purple? Are you eating a lot of green foods or taking spirulina, chlorella, etc.? Are you supplementing iron? Any of these could do it.
Green stool can also occur when there is a bacterial imbalance in the intestines. Have you had any digestive problems, candida or parasites? Die-off from critters can exacerbate the signs of this and lead to green stools. Iodine could be causing die-off.
Bile is green. If there is a heavy dump of bile and the stool moves quickly through the intestines it may result in the green color.