I was at a hotel yesterday on business and I passed by the hotel's swimming pool. The water was clear, but it was green as grass. No algae was floating on the surface and except for the green color the water looked clean. My question is how to you know when you are first starting to get algae? I would have thought that the first sign was cloudy water. But this water was clear--just green. Is the change to green color the first sign of algae? Does a green pool necessarily have algae? Was it something else?