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.
Most women should aim to consume between 1,000 and 2,000 milligrams of calcium daily. The Mayo Clinic notes that some men may benefit from calcium supplements as well. Men over age 50 should consume in the range of 1,000 to 2,000 milligrams per day and 1,000 to 2,500 milligrams per day for men under 50.