Typically, pool water turning greens indicates algae growth. If the pool takes a mild green tint, you are in luck, because it means you caught the problem early on.
It also means that there is undetected chemical imbalance. When chlorine levels are too low in the pool, algae can attach to the pool surface, or float and grow.
Next, super chlorinate the water using pool shock. This will kill all bacteria and algae, but depending on the pH levels, the water may get cloudy to very cloudy. The pool water will clear up once the filter is running.