White marks on dark painted wall

[bathroomadvice]

Hi these white marks on our bedroom walls are driving me mad. I have washed down the walls with detergent and then tried a vinegar mix but it keeps coming back.
Any ideas what other products I can use? I guess using a diluted bleach will remove some colour? I try to switch the dehumidifier on regularly but my son forgets a lot.

What do you think the marks are & why are you thinking the dehumidifier would help? Are you thinking they are a bloom of mould? In which case the cause needs to be addressed before they stop.
When did they appear? Is it not just mostly a residue of whatever you’ve wiped with? I can see the wipe marks so the area isn’t totally covered.

Is it mould OP? Hard to tell from the photo if I'm honest, it's not coming through as great quality on the app!

They could be salts coming out of the plaster. Check first of all with a damp meter (quite cheap on eBay) if you have damp, if not, spray with stain block and then repaint over it. It it's damp you'll need to deal with the source of the damp first! (eg is there a bathroom on the other side of the wall)

Is it a newly plastered wall? If so, have you primed it first?

To answer some questions, thanks for the responses!
The room smells damp so I thought it was mould. Hasn't been plastered recently we repainted the walls this colour 3 years ago and for the last two winters the walls go white like this. Although the worst wall is internal not external?
Maybe it is some residue from a cleaning product?

you may have damp rising through the walls so check with a damp meter.

Seems weird that it's been happening in winter! Could it be the salts coming through? If it wasn't painted / plastered / prepared properly could it be the water coming through, drying and leaving the salt deposits behind?

Can you get a proper primer, prime and repaint the wall? Like gardz?