UPDATE
I checked how the heatsink was mounted and the thing you pointed out on the picture is the tip of a screw most likely, the problem is i can't remove it because the screw goes through the mobo (obviously) and the nut of the bolt is on the side. You see this pc ws basically gifted to me a year ago ( or otherwise it would have nvidia and intel in it over amd but i digress) and i never took it apart. I also have a really cheap and old case, so getting the motherboard installed was a bit tricky, according to the IT guy who installed it, so i have no idea where to start even, not that i really want to take it apart entirely. So for now the grease stain remains and it's cause unrevealed, but sometime in the near future i'll take the PC to the IT guy and have him dismantle it.