Yoga exercises could stimulate the passage of (green) food and/or (originally green) bile through your bowel, and then either the food or bile would color the stool green. If stools are loose and contain undigested food, you are losing nutrients this way.
Excessive use of magnesium supplements can be toxic. The upper limit -- the highest dose a person can take -- of magnesium supplements is: 65 mg/day for children ages 1-3. 110 mg/day for children ages 4-8. 350 mg/day for adults and children ages 9 and up.
If you decide to take supplemental magnesium, the recommended starting dose is 400-500 mg/day in divided doses. The limiting factor for most people is diarrhea. If you are on a migraine preventative medication that is constipating, like amitriptyline or verapamil, this might be a plus. Itâs always wise to check with your doctor before starting on magnesium, especially if you are on medication.