For some reason, I didn't copy the below paragraph to my earlier post, when I posted. Whether it is relevant, I will leave it up to you to decide.
I have these same concerns and use Jegs' Fuel Jugs to fill up my various fuel tanks, Motorcycle/Generator/TV. Years ago I found that these Jug containers holding fuel attract contaminants for some reason or other. Using a filling tube in a cap to these jugs, I did like Reddog1 did with his generator; I picked up a fuel filter from an Auto Parts store and put it in the middle of my fuel filling tube. That way I can filter a number of vehicle's tanks, before it arrives at their filters. I still want to do an in line filter for my Honda Generator and it is on my growing "to do list."
I use a plastic 3-gallon boat fuel tank to fill my Honda. I use a glass see through cleanable fuel filter in the hose between the boat tank and Honda tank.
The stuff was in the hose after the Honda OEM filter, and before my add-on filter between the Honda OEM filter and carburetor. If the stuff was from my boat tank, it would have had to pass through the boat hose filter, Honda screen and the Honda OEM filter. I am not convinced it did that.