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.
My doctor just put me on a Calcium supplement that has Vitamin D, and Magnesium and other stuff. He has me taking 600mg twice a day with food...one in the morning w breakfast and one at night. He said only take one at a time because it would not absorb the extra if I take them at the same time.