Green/Brownish Staining around the waterline

[Blackcat]

Hello,

I am cleaning a pool which has green staining around the waterline, it's a plastic membrane lining. Everyday I clean it with a sponge and pool cleaner (like liquid fibreglass cleaner). I think that the stains are caused by sun tan creams & oil but although warned the guests continue to use it. Short of actually showering them down myself does anyone have any ideas how to remove the stain? or at least make it look better?
Thanks