Stain at water line after removing cover

I have an in-ground pool. The walls are 3M colorquartz product (which I believe is marsite based but not sure). When I remove my winter cover, I have a faint stain line where the water line was. This water line is not under water when the cover is removed because the water that was on top of the cover displaced some of the water in the pool. I was wondering what would be a good way to remove this stain line. I was hoping to do it (thinking it would be easier) before filling the pool the rest of the way. Thanks.