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.
Because Calcium can cause constipation, it is necessary to take 500 - 1000 mg of magnesium at the same time you take Calcium. You should space out your intake of calcium over Take only 400 to 600 mg each time. Also take some time-out when taking calcium and other vitamin supplements.