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.
Magnesium taurate is recommended in particular because it is more readily absorbed and better tolerated than other magnesium supplements. Begin with a daily dose of 375 mg, taken at bedtime. Follow that regimen for a month. If little improvement has been achieved, increase the nightly dose to 500 mg.