Yes, it does: you are going to struggle to really get rid of it in that case, because you can't remove the substrate on which it's growing, which is the plaster. It's porous to a degree, which will have allowed the hyphae of the mold to get down into the material.

I don't usually deal with that type of construction, I deal with drywall, but since the impacted materials can't be removed, I'd focus on killing it, sealing the wall, and not allowing it to return.

I recommend covering up any HVAC returns in this space while you remove the wallpaper and clean the walls. I suggest a quaternary ammonium to clean and kill, and then when the wall is completely dry, apply a coating that is designed to prevent mold growth. There are many brands, PPG makes a good one. Kilz makes a poor one.

Then do your best to ensure you keep your home within bounds for air quality to prevent condensation from occurring indoors. I don't live in a cold climate so I'd need to look up some additional details regarding that, but you can check ASHRAE for guidance.