keep furniture away from the wall. wipe the windows down every morning to remove condensation. if you can, open the windows for 15-30 minutes after you wiped them down. get a fan and aim it at the wall, just enough to keep the air moving.

This, and maybe place some (chemical)dehumidifiers in the room.

You need to figure out why the wall is getting wet. Is there extra rain hitting that wall outside? Is there a leak inside of the wall? you have to get rid of the moisture source, or the mold will keep coming back.

Do not treat mold lightly. It can be really bad on your health. There is a subreddit called toxicmoldexposure if you want to see what the symptoms are.

You should call a specialist.

You should not put wallpaper on exterior walls. There's mold growing behind it now. The drywall should be removed and replaced. Do not put vinyl wallpaper back on these walls or it will happen again.

It's an exterior wall with wallpaper on it. That's acting as a vapor barrier. There may or may not be additional water intrusion, but there doesn't need to be for this to happen. Water is condensing between the wallpaper and the wall bc of poor insulation and temperature differentials from outside to inside, that's my assessment.

Thank you. Does it make a difference that it’s plaster then brickwork behind the wallpaper?

You most likely have a leak somewhere it depends on where the room is.
It could come from a leaky roof, pipe, condensation.
You should address where the water is coming from BEFORE you remodel the room or you will be throwing money away.

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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.

If so, that would be happening on all similar ext walls with wallpaper.

And would be happening worse closer to the ceiling as it’s hotter up there.

If not, then I think there is more going on. Certainly it warrants closer investigation.

Thank you. That’s really helpful.

No, not necessarily. It matters which cardinal direction each wall faces.

And what you've said about it being more pronounced up near the ceiling does not square with my understanding of the situation. It matters where the air supply diffusers are directed, the position of the doors and windows, and other factors like insulation.

I'm not saying there's definitely no additional water intrusion, but I'm not understanding the reasoning you're using.

Edit: thanks for confirming that the "reasoning" you're using here is... None. Suspicion. Alarmism.

I am 100% certain that subreddit is full of pseudoscience.

OP, please do not go to that sub and work yourself into a frenzy.

No, it's not good to be exposed to mold. However, the most likely outcome is allergies. Like, regular allergy symptoms. Many insane "medical conditions" are attributed to mold exposure without evidence.

Again, I'm not saying mold is good for you. I'm saying alarmist pseudoscience doesn't help anyone.

According to the FDA there is no danger to long term mold exposure.

So that’s nice.

BTW if you believe the FDA on that kind of thing, I expect there will be some good deals on Ohio housing in the near future.

Obviously a subreddit full of whack jobs is a far better source of information.

But I'm glad you disclosed where you get your info from and what you consider a reliable source. I'm sure that will help OP properly contextualize your suggestions.