From a bit of googling, it appears that there is a 'pink slime' called methylobacterium that often grows in showers. Could it be that?
Molecular Analysis of Shower Curtain Biofilm Microbes:
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Methylobacterium spp., especially Methylobacterium mesophilicum, have been cultured from pink-colored biofilms of wet environments such as automobile air-conditioning systems, printing paper machines, and dental unit waterlines (3, 28, 44, 53). Considering the abundance of Methylobacterium spp. rRNA gene clones detected in this study, such organisms likely are responsible for the pink color of some shower curtain biofilms.
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