Above swimming pool

Take a pool water sample to your local pool dealer they will probably have you use an algaecide.

You need to check your Chlorine and pH levels. A quick fix is a shock treatment, but you need to find the culprit and correct the problem or you will have it again. Here are the recommended guidelines:
Chlorine:
To maintain the proper level of residual chlorine when bather load is heavy, shock your pool once a week with a 3 to 5 times higher than normal dose of chlorine.
Chlorine is broken down by the sun's ultraviolet rays. Using cyanuric acid to stabilize the chlorine in your pool at the beginning of the swim season will help you maximize your chlorine's efficiency all summer long.
Filters remove suspended particles from pool water, but don't control bacteria or algae. Maintain a residual chlorine level of 1.0-3.0 ppm (parts per million) to kill bacteria and/or algae present in the water.

PH LEVEL
The ideal pH for pool water is 7.5. A pH range of 7.4-7.6 is considered acceptable.

WATER CLARITY
When opening your pool at the beginning of the season, run your filter around the clock until the water is completely clear.
If your pool water appears green or has an unpleasant odor, the problem is probably caused by algae. Test for proper chlorine level, and consult a pool professional if the problem persists.
Keeping your filter, pump, lint trap and skimmer baskets clean and in proper working condition will help ensure that your pool water stays sparkling clear.
Pool inlets should be adjusted so the surface water is moving in a circular direction.
Make a habit of checking and emptying skimmer and pump baskets regularly.

Hope this helps. Susu

You also need to add alge prevention, once a week or after a rain, about 3 oz only.