Deegeelion

Hello, bizarrely a yellow waterline stain has appeared around my layzee spa Paris. It is 4 months old, well maintained and water is always clear, however the staining seems to be within the inflatable and not external as nothing is coming close to removing it- I have tried a chlorine burst with no luck, absolutely baffled me how I can get rid of it. I have attached a photograph. If anyone can help, Thank you. ( for some reason photo has spun, but hopefully you can see what my problem is)