What happens next?

How to treat algae in a swimming pool
Test and adjust the pool's ph.
One of the main causes of algae growing in a pool is if the ph of the water gets too high, because this prevents the chlorine from killing the algae. Collect some water from the pool and test the ph levels with a test kit.
To lower the ph, add some muriatic acid or sodium bisulphate. To increase the ph, add sodium carbonate.