just pull it off, avoid touching the pins though.
that must be old thermal paste. or glue, but it wouldn't matter how old it is if it dries within 10 minutes because it could be 8 hours old or 8 monthes old and wow, major rambling....
just pull it off slowy i guess, nothing else you can do.
or if you dont care about the processor just shake the heatsink violently till it comes off.
best of luck, yours truly
-Joe