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.
Your doctor will tell you how much iron you need to take every day. Typically, adults need to get between 60 and 120 mg of iron a day. The best way to take the supplement so that you absorb the greatest amount of iron is to take it in two or more doses during the day.