The solid core of pus in a pimple. The moon rock must be excavated for the pimple to recede entirely (not just scraping off the top), although getting to it can be difficult. Once removed, the pimple should excrete a bead of blood mixed with clear fluid, then begin to recede.
The best way to expose moon rocks is to either squeeze the pimple with two fingers in a traditional fashion, or use the tip of a pointy knife to apply pressure below it. Either way, the solid and semi-solid pus should pop cleanly out of the pore.