Fix green pool water because it is likely caused by algae, which is dangerous to your family and friends and will harm your pool equipment. Green pool water is a visual clue that your pool water may be contaminated depending on the specific cause. It reflects significant growth of micro-organisms in your pool water. Swimming pools have restricted water volume and continually pick up micro-organisms that have nowhere to go. If left untreated, they will proliferate and turn your pool water green. Apart from algae, these micro-organisms also include bacteria, fungi and germs.