The same thing happened to my 40th. I put the same cover on and didn't realize the plastic was sitting on the bumper. I left it like that in the sun for 3 days and my guess is that the plastic got so hot it baked into the clear coat. What I ended up doing was cover the plastic with some material from the bag that comes with the cover. I mention this on this following thread:
Nizmo Car Cover
The damage didn't look too bad and only noticeable up close.