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.
The Recommended Daily Allowance (RDA) of magnesium for young adults is around 400 mg/day for men and 310 mg/day for women. For adults over 30, the RDA is 420 mg/day for men and 320 mg/day for women. However, despite how easy it is to find magnesium, studies have shown that between 68% and 75% of American adults are magnesium deficient.