Dirt...
I have noticed this on my car, too. I suspect that rather than mold, what you are seeing is road dirt, composed of soot, dirt, oil residue, brake and tire dust and anything else around, that is kicked up into an aerosol when it rains, and flows into the fuel door. It accumulates there because that part of the vehicle isn't washed very often. I cleaned it off my car with car wash soap. I suspect that a good coating of wax would make it easier to remove. Might also be worthwhile to confirm that the fuel 'box' drains are working. Cars here in SoCal (where it doesn't rain very often) don't have much of a problem with this.
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