GRP tanks
I have seen something similar in a gascolator which was fed by a GRP tank where the fuel had been standing for approximately a year (actually the contents of the gascolator looked like golden syrup, I scrapped the tank, the previous owner had quite happily flown the aircraft with this fuel and crashed it). One piece of advice I was given was if the materials from which the tank was made are still available ie you made it yourself and you have some of the resin left was to make up some test strips. Then dunk these strips in a jam jar of the mogas you are currently using and observe. Repeat frequently, Mogas formulation varies according to supplier and season