What is this!?

[WhatIsThisOnWall]

I've only just noticed this today. For reference live in a studio flat and have been here since 2016. The paint is all crumbly at the top, you can see where I've touched it and some has just fallen off completely. It's an outside facing wall next to a window, none of the other walls have this.

Forgot the image

....

It’s a water leak of some kind. You need to get that checked properly.

Looks like you’ve got a leak from somewhere. Flat above have water/radiator pipe above you? Outside drain blocked or broken? I’d ask neighbours upstairs first. We had a slashed radiator pip upstairs and it took several months before the damp and water came through the ceiling downstairs.

Looks like a leak to me or water is getting in from outside.

Is the down pipe the other side of the mark? When it rains does water run down that part of the wall?

If it does then there's your problem could be a simple fix of clear your gutters or something more.

It's appeared really suddenly, I've literally only just noticed it today... so I would guess a leak. It's a bit late to knock above? Or is this urgent enough to knock now? Can this wait until the morning? Never dealt with a leak before.

Is the down pipe the other side of the mark? When it rains does water run down that part of the wall?

I actually can't recall off the top off my head if there is a pipe on that part of the outside wall, I'll take the bin out quickly and take a peek before bed.

Did you get to the bottom of it OP?

I did message and voicemail the letting agent and they haven't answered as of yet. Going to follow up tomorrow. It hasn't become any worse...

Checked outside, no pipe running by the wall. There are water type marks on the outside of the wall though! Haven't been able to get an answer from the upstairs flat when I've knocked.

They are sending someone to look. Although there isn't a pipe running up the wall directly, it's further along on a different wall, if you follow it up there is a gutter going along the top, I hadn't been able to see it due to the angles and I've not got great eyesore, so they are sending someone to take a look out there.

However, of course it looks horrible inside. When the source of the leak is fixed, what can I do to make this look better? I can't just paint over it :(. I was going to paint the flat anyway as it hasn't been decorated in over 7 years and the walls are not great, was going to try and match it to existing colour, sugar scrub and fill it some tiny holes, but what about this wall!? Any help

See what it looks like when it dries out, you may have to knock out the 'bad' plaster and re-plaster or fill the damaged area. Overfill let it dry rub down level and then paint.

It could be a leaky roof or the gutters need clearing/mending.
There’s no point in doing anything inside until the source of the water is fixed, as anything you do will fall off again.
Once it’s mended, depending how bad it is, you may need it replastering. Or, you may get away with rubbing it down with sandpaper, painting over it with a SBF primer, then a layer of damp stop paint, then emulsion.

https://www.zoro.co.uk/shop/tools-and-machining/interior-paints/damp-seal-paint-500ml-plcds500/p/ZT2596674S?utm_source=google&utm_campaign=pla%2B%7C%2BTools%20%26%20Machining&utm_term=ZT2596674S&utm_medium=pla_css_3&targetid=pla-1907556790985&loc_physical_ms=9045119&dev=c&gad_source=1&gclid=EAIaIQobChMIuL3hi8fDggMVBsXtCh2zvAAHEAQYBSABEgLxs_D_BwE

Your landlord will need to fix the inside - they will need to paint over with a stainblock paint first though once this is dry