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.
Maximum daily dosage may differ depending on your age and condition. Your body's calcium needs range higher as you get older. For most adults, 1000 mg of calcium per day from food and supplements suffices. Adults above 50 year of age require about 1200 mg of calcium per day.