I have an inground pool that has a vinyl liner and is approximately 8000 gals.. Recently I have noticed that the vinyl liner has become discolored a yellowish color below the water line. It will not come off with brushing but will come off with hard hand scrubbing. Pool water is perfectly clear. Just the discolorization on the liner and steps and skimmer plastic. Not a heavy discolorization but noticeable. Any ideas on eliminating?