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.
For magnesium deficiency: 3 grams of magnesium sulfate, taken every 6 hours for four doses, has been used. A 5% solution of magnesium chloride has been used by mouth daily for 16 weeks. Magnesium-rich mineral water (Hepar) containing 110 mg/L has also been used. 10.4 mmol of magnesium lactate, taken by mouth daily for 3 months, has been used.