I wouldn't use acetone or MEK. It looks like mildew, if you're not careful with solvents, what happens is the solvent melts the surface of the material & mixes in the muck from the mildew & bonds it to the surface. The solvent evaporates, the surface sets & hey presto, a nice mildew paint job. I'd attack it with strong non-solvent cleaners like TFR or bleach.
Rule#2: Never argue with an idiot. He'll drag you down to his level & then beat you with experience.
Rule#3: Tha' can't educate pork.
Rule#4: See rule#5