I would open an AMD WARRANTY REQUEST from here : AMD Warranty Request Form | AMD and upload your Photos plus all the other required information and see if they say it is "Normal" to have those stains.
From what I have read, if the CPU came in a sealed box from AMD, the stains could be from the manufacturing process. Most say, see if you can clean it with Isopropyl Alcohol and it should wipe off.
Since you mentioned it is working normally then I wouldn't worry about it unless you feel it is a used CPU sold as a new CPU.
But to ease your mind there is no harm in opening a AMD Warranty Request just to double check with them.