I used a paper towel to remove the stock thermal grease from the heatsink AMD gave me. I then apllied an Arctic silver knockoff grease because the chemical components were the same!
the thermal grease already with heatsinks is usually a perfect quadrilateral. Mine was. They use machines to apply that stuff. It's entirely possible it's grease, but that's not the point. scrape it off either way. Use a credit card on both if you want, but use a lint free towel to wipe the rest of the stuff off.