What the hell is this on my 1ds mkiii sensor?!

All,

I just notice this smear when I tried to do a routine clean. I have used Eclipes E2 and Pec Pad more than 10 times to wipe it, but it is still there. Just now I tried using Eclipse (stronger but not recommend by maufacture) twice. Still no luck. It seems can be moved but cannot be cleaned! Any suggestions?

Thanks.