Im hoping I can get a few other to chime in here. So far I have tried everything I can think of except straight muratic acid and nothing will take this stuff off. I pulled the top rails off and its not stain from rusting screws. It has not gotten any worse since pool opening.

Could this be it: last year I used a floating chlorinator and at times it sat in one spot over night stuck under the solar cover. Could the damage be from excessive clorine contact?

Other than looking unsightly the liner seems ok and I am not experiencing any water loss. Im not replacing it until I have to.
Thanks for all the help
John