I am just assuming but is it due to an aftermarket exhaust? I'm guessing that the heat from the exhaust did this. I have seen alot of people not use that plastic piece when using an aftermarket exhaust. My suggestion would be to use a heat gun to get it off. Try heating it evenly, get it hot but not too the point of melting. Basically I mean try to heat it up to remove it as one piece. You woulndn't want it to get too hot and become a mess. The razor knife suggestion is good but would just take a long time. If you go with heat and you have left over residue use the razor or steel brush maybe some sand paper. If it ruins the finish on the exhaust and your worried about looks use steel wool to even it back out.