Green veggie will sometimes normally cause green stools especially if eaten in abundance in any given week. However a runny stool is indication of inflamation in the stomach and can lead to dehydration.
green colored stool my occur twice in row but should not continue if not eating green veggies or fruits by the bag. See a Dr. if this lasts more than 3 days, this could be gallbladder trouble, if pain is on the right side of upper abdomen. Good Luck