Does anyone know what this is?

[Worried789]

I’m not sure if I’ve got the right discussion board here! We recently had our house fully renovated (new plasterboard, plaster, painting etc). These spots have started appearing in our bathroom above the bath when it gets wet - I was really worried it was mould but it disappears again once dry. It’s only in certain patches on the wall (our little one accidentally sprayed water all up the wall and half of it just has droplets and half has these grey marks).
Could this be mould?

Might sound like a silly question but was proper moisture resistant bathroom paint used?

Thank you for your reply. It was Farrow & Ball Modern Emulsion paint - meant to be good for bathrooms and mould resistant but not an actual anti mould paint in don’t think!

Ok sounds like that paint should be fine. Did you paint it yourselves or have a decorator? Just thinking if someone else did it you could send them a photo and ask.

Hmm. I've been a fan of F&B paints for decades but recently had some problems.
I used the Modern Emulsion for the first time in a kitchen and found that the coverage was absolutely dire. Really thin and patchy, even after two coats.
I abandoned it and went for the Dead Flat Emulsion, which was considerably thicker and gave complete coverage after two coats.

It looks as if, possibly, your paint is so thin and patchy that the plasterboard (or whatever the substrate material is) is exposed in places and the water vapour is starting to penetrate it.

Thank you for the replies! Yes I wondered if it might be that it needed another coat in places.
Our builders used a decorator they knew, so I was keen to get an idea of what it was before speaking with them about it in case they dismissed it as nothing to do with them!

Painting isn't rocket science.
If you take a really good look at the paint work you can easily see if there is sufficient coverage or if the substrate is still visible.
Have they painted over raw plasterboard? Was a mist coat applied? If so, which paint did they use? Specifically, brand and type. Eg Dulux Trade emulsion.
Did they use the primer/undercoat specified by F&B? (frankly I don't rate the F&B primers/undercoats, but you have paid for that so need confirmation it was used).

Ask questions, don't agree to anything. Something has gone wrong.
Happy to advise if you need help.

Thank you this is all really helpful.
the plasterboard was plastered, then a mist of white paint applied then the F&B on the top. I’m not sure about a primer but I’ll ask.
thanks very much again, helpful to know what I should be asking!
The house was left soaking wet for ages as the roof was replaced during a period of bad weather (before plasterboard went up) so in the back of my mind I’m always concerned about it being mould coming through. But if that doesn't look like the case that’s a relief.

I had something similar after a bathroom refurb—turned out it wasn’t mould, just the way the new paint reacted to water. If the grey marks only show up when wet and disappear once dry, it’s probably not active mould. Mould usually sticks around and gets worse over time, even when dry.
It could be uneven absorption in the plaster or a patchy paint seal—especially if the wall wasn’t fully dry before painting. If you’re worried, you could try gently cleaning the area and keeping it dry for a few days to see if anything reappears.
I’ve been using a free visual scoring tool to check mould risk from photos—happy to share if you want to double-check the area. But from what you’ve described, it sounds more like a cosmetic issue than a health one. Hope that helps!