Green water is an outbreak of microscopic algae. It's caused by an imbalance in your tank. You have too much light, or too nutrients (fish waste or excess food rotting), or both.
To prevent it: Don't overstock your tank, feed lightly, reduce lighting. Or add fast growing plants (if you have the correct lighting to do so).
To cure it: reduce the number of fish in the tank, feed less, daily 25-50% water changes to remove the disolved organics, reduce light period.