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 dietary allowance of magnesium for breastfeeding women is 310 milligrams a day for those under 30 and 320 mg a day for those who are 31 years old or older. In addition, some other medical conditions may require a person to take larger-than-normal amounts of magnesium.