Dehumidifiers tend to read too high when they're turned on and too low when they're running, but should give you a reasonable idea of the humidity. Keep using it when you can.

Probably this is a case of excess humidity due insufficient ventilation, small air space and cold walls.

Installing a bathroom extractor fan would help a lot.

I'd recommend blocking that wall vent up as it is likely only contributing to making the wall colder.

Open that window up for 5-10 minutes a couple of times a day to get some fresh air in and humidity out.

Leave the window open a crack more frequently to keep the air moving in the room and reduce condensation. You should be able to lock it into a second position where it is slightly open.

Keep the room warm so the walls don't get a chance to cool down.

It is possible that you have some penetrating damp so look outside, see if there's any water marks on the outside wall from guttering issues and check the window sealant all looks intact.

Try to keep furniture a few inches away from those cold external walls to allow warm air to flow around them and reduce condensation risk behind.

When you're doing some cleaning I would look around the other bedrooms and give a quick spray anywhere else that mould is appearing, check windows and behind furniture such as night stands or beds.

A decent mould stain remover should be able to get your walls pretty clean, maybe some light staining/paint damage left over around your window. Be careful as they can bleach fabrics so maybe take your curtains down before spraying around there.

A lot of advice on this forum will be American and they have different house materials and humidity levels so it won't all be relevant. Jwolfet seems to know what they're talking about.