Pink algae does wipe off, so I don't know what it could be. Do you have any problem with orange dust that may have gotten under your cover and stained? You can tell if it is a mineral stain by rubbing some ph down, or a vitamin C tablet on it. If that takes it off then it is a metal stain. If it is algae then chlorine should kill it. Some things like leaf stains, and stains from vegetation take some time to lift off, but with the regular use of chlorine in your pool you should see it fade over time. Hope this helps.