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.
It is best to take magnesium supplements with a meal to reduce stomach upset and diarrhea unless otherwise directed by the product instructions or your doctor. Take each dose with a full glass (8 ounces or 240 milliliters) of water unless your doctor directs you otherwise. Swallow extended-release capsules whole.