When I opened my pool this summer, my spa ( where the water had been lowered and 2 pillows used to support the cover, had a brown stain above the water level. I thought it was organic but after many shocks, the stain remains. Also found a little brown stain at spa light. Other than that, the floor and walls below the water fill are fine. Not sure what this is and how to remove it and to prevent it in the future.