Green swimming pools are caused by algae. This happens when there are chemical imbalances in your swimming pool water. The main reason for a green pool is the lack of chlorine, but it is good to remember that high pH can result in poor chlorination. Often times, high pH is an indication of too high of an output from your pool salt chlorinator.
It is always important to keep your pH within the range of 7.2-7.6. This is where water remains neutral and chlorine works best.