Why is my cloth-like (polyester) shower curtain liner molding so quickly?

[Edited after typing my question: I just checked and my shower curtain is 100% POLYESTER] I recently moved and purchased an almost cloth-like shower curtain liner (it is not thick plastic like my old one). I have lived here for about three months now and the bottom corner where the shower head is aiming at has... show more [Edited after typing my question: I just checked and my shower curtain is 100% POLYESTER]

I recently moved and purchased an almost cloth-like shower curtain liner (it is not thick plastic like my old one). I have lived here for about three months now and the bottom corner where the shower head is aiming at has already started to brown pretty severely.

a) What is the science behind this "molding" process and why does it seem to enhance with cloth-like materials?

b) What is a good way to clean this type of shower liner?

Thank you
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