Have a look at the boat washing and cleaning products from http://www.farecla.com/#/6
They have specialised cleaners fro removing difficult stains from fibre glass boat hulls like waterline stains which must be similar bacterial stains, or worse, as you are experiencing.
Use a cleaner that's formulated for boat fibreglass and follow it up either with a good quality boat wax containing carnauba wax, or use a glaze to provide year-long protection. There's loads of information at http://www.boatsheen.com
------------- the only silly question is the one you do not ask.