What could cause this on a window ?

[Ceebeegee]

As the title suggests.. .. what would cause this wavy discolouration on the window ? Picture attached . It's in between the panes so I cannot wipe it clean

Oh wow. That’s quite amazing.

I Think you have accidental harboured some kind of science experiment.

I'm only guessing, but looks like moisture has got inside and its mould growing!

It looks as though some moisture has got into the window and then either a mould/fungus has grown or there is mineralisation.

Yup. Looks like mould.

Eek. It was on a property we viewed this morning it has been empty for months and has been neglected.
Thank you for the posts.

Seal's gone on the unit from age/sun. It's likely that all of the windows and any glazed doors will need doing as well - and it increases the chance of them popping in winter from the difference in temperature between inside and out.

The seal has blown. If the frames are in good condition, you can just replace the glazed units

Panel has definitely blown. When that happens, liquid gathers inside- it looks like this has become something else. Not overly expensive to fix- a few hundred to replace the pane- unless it has damaged the frame, then you are looking at more.

I would worry that this is indicative of lack of maintainance, though!

I would check all the other windows carefully as they were probably all done at the same time.

The maintenance at the house is odd. It has a newish air source heat pump and a cylinder cupboard that looks like the inner workings of a spaceship. Brand new radiators throughout. New kitchen approx 2014 and still in very good condition . But some rooms still had polystyrene ceiling tiles.
I've seen the installation receipt for the windows and doors - the windows were installed in 2010 throughout, the rest look okay , its just this one that looked odd. It doesn't show up on the picture but in some angles it looks like oil slick / iridescent. Just haven't seen anything like it before.
There's a lot to consider before making an offer on it, I'll take into account that this window has definitely failed.

Oil slick/iredescent usually just means the unit has blown (I just replaced one unit that looked like this)