What causes a green pool?
The number one cause for green pool is algae. Algae will start grow in your swimming pool if there is lack of sanitizing agents in the water. Chlorine is by far the most popular and most effective sanitizer that's being used in swimming pools today.
If you have a green pool, you most likely have little or no chlorine in your water. Maintaining a proper chlorine level is the key to unlocking a crystal clear pool.