Actually, when you have pools like the country club pool I service locally, they tend to maintain low levels of algae most of the time. I don't know why that pool is so prone to mustard algae, but apart from that it has a damaged finish with many, many cracks and crannies where algae can survive regardless of chlorine level. I've been in the pool with SCUBA, when the chlorine level was VERY high, and still found live algae in areas where the tile grout had eroded. None of that algae was visible from the surface.

But, when you have a persistent algae population, it will create a persistent and higher than normal chlorine consumption. This is the pool in which I'm experimenting with PO4 removal, and the persistent higher than needed demand is one reason.