John, from what you and Carl said, I'm thinking that the screws that hold the top rail down are oxidizing and what you are seeing is rust that has dribbled down the wall. The ascorbic acid (vit C) will liberate the stain, Mr. Clean Magic Eraser also has been known to work. I'm not a large fan of the pool store tile line cleaners - I just use a paste made out of baking soda on a green kitchen 'scrubbie' to remove the oils and dirt that accumulate at the water line (though sometimes a weak acid solution is needed)

I sincerely hope that this is helpful to you and that you get to enjoy the pool more than you have to work on it (in CT - we are starting a 4 day heatwave with temps over 90 - maybe now the condo association will let me put in a pool )