We've got these 3 stains on the bottom of our liner which look like a very large alan wrench was left in the pool for a while. This is our second summer in our new-to-us pool. We've only ever used the TFP method to take care of our sparkling oasis with never any problems. The previous owner... not so much.
I don't believe this stain is from anything other than the tool rusting there. (Last summer I found the tool that fits this very pattern in the bushes.) I'm not sure what to do with it. Leave it? Try some vitamin C?