What should I do about this

[scx18x]

So I moved into a new home 6 months ago. Recently this has appeared on my kitchen wall could someone tell me what this is? My landlord is useless and dosnt seem to bother

Is the wall wet? It looks like damp and some kind of mould?

What's behind that wall? Does it open, there's a definite line. It looks like whatever's behind there is leaking.

Is that a plasterboard wall? It does look as if there is a leak, what is above it?
The red mould is something that seems to appear with water on grouting, it would need scraping out and a better quality grout or sealant used but no point unless the leak is fixed as it will just grow back. You can clean it off with a mix of bicarbonate of soda and white vinegar but I would leave it until the problem is fixed. Water splashes can also cause that.

What is on the wall to the left of the damp patch backing onto it? Looks to me as though it could be a boiler? There are pipes above it at the same height as the damp and it's clearly an inspection panel on the damp wall.

Worryingly that looks like a boiler to the left and maybe a consumer unit (electrics) or an extractor fan above the window. He is being an idiot not dealing with this.

It looks like water damage and the horizontal line possibly indicates a noggin which is why the water is running across the wall and then down the joist.

I would take photos of it with a ruler up against it so you can see how wide the spread is. Report it back to your landlord immediately.