It looks like leftover bits of the thermal pad. Try removing it with some lighter fluid. It takes a few seconds for the fluid to start dissolving the hardened material, so don't scrub at it. Just put some fluid on a clean cloth that you're not going to use for anything else (very important, mother's tend to have fits when when you use their good cloths), and hold it to the die for about 30 secs. Then start wiping gently at the die. The die should look all blue after you're finished. Then examine it very closely. Check to see that there are no chips taken out of the die. If you can, see if you can get a close up of the CPU die. It's a little hard to tell in the pic you posted.