You are seeing water vapor - just like fog. The air just (within a few inches) above the pool is warm because the pool is warm. This allows for high humiditiy (warmer air holds more moisture than colder air). As the hot, moist air rises away from the pool, it cools and the water condenses and forms the fog or mist you see. You see it at night or very early in the morning because this is when the air temperature is cooler.