If your inground or above-ground pool water is green, murky, or stained with color, you may have an algae infestation. Swimming pool algae is a nuisance for pool owners working to keep their water clean, clear, and ready to use. You might be particularly concerned if you observed this while using the pool, leaving you with the nagging questions: Is pool algae dangerous? Can you swim in a pool with algae? Where did it come from, and how do you treat swimming pool algae?