What causes swimming pool water to turn green when it is shocked?

If your pool water turns green after it is shocked with chlorine, it is too basic. Algae can thrive there now and it is green. Add pool acid (it may require several gallons) and check the pH of the water frequently, adding more, if needed. If it gets too acidic, you can add some baking soda (which is basic) until you reach the desired pH - slightly below neutral. Neutral is pH7.0

answer : if you used well water you have metals in your pool take a test water sample to your pool dealer and have it checked for metals and he will advise what to do next