It's the inner part of the plastic, so you could paint thinner it and wouldn't be too noticeable! Or some rubbing compound may take it off without deforming the plastic in there. It's already smooth inside the wheel well so maybe even try some 1000 grit sand paper. I would try the pressure washer first like Sue advised, because there's a good chance it wouldn't even stick or cure well to dirty/oily plastic anyhow.