I have a couple thoughts about what it might be, but before guessing can you feel anything odd on either side of the glass that might indicate what side it's on or have you tried scraping it with a razor blade or any kind of cleaner to see if it affects the stain?

Are there any information markings in any one of the corners of the glass, if so what does it say?

And can you take a closer picture of the lower left side of the window?

I am not confident that's ever coming off, but let's see if we can figure out what it is first and then maybe give you some options.

Hi! Would love to hear your thought on what this could be!
There is nothing to the touch on either side of the glass. I’ve tried using windex and acetone to remove but it had no affect. I haven’t tried a razor blade, but since I couldn’t feel anything I didn’t think it would help.

Here are some more pics of the information marking and a close pic of the left side.

Insulated Glass Seal failure, commonly called "fogged". New insulated glass unit is needed.

Thanks @millworkman!

In your first post you mentioned that it doesn't appear to be dual paned window...can you tell if there are two lites or not in the window?

The second picture in the second set looks like there are two. Can you confirm?

Are you in Florida?

If you look at the lower edge of the first pic @oberon476 I certainly believe that is a dual pane unit.

So it was hard to tell from the bottom corners but looking more closely it does appear to be a dual pane. Here is a closeup from the upper corner.

I’m in NY but we just moved to this house so not sure how old the window itself is and when it started to happen.

Yep, I was so interested in the pattern of the failure that I missed it completely. The last picture is certainly definitive.

The pattern itself is very reminiscent of a particular resin laminated glass failure from the mid 1990's and I recall that Andersen might have had a few of those products at one time, but I had never heard of an Andersen unit that had failed for that reason - again given it was a long time ago.

When I saw the "crack" pattern in the third picture it really made me look closer at the pattern because that was one of the tell-tales of the resin laminated glass failures. I can't remember ever seeing the "cracking" pattern from a simple seal failure, so that IGU still has me interested, but I agree since it's an older IGU that it's simply seal failure - if a very interesting one as a quality guy if not to the homeowner who has to replace it.

I would guess that the IGU is from the 1980s or even earlier and it absolutely needs replacing.

Interesting! I learned something new :)
I’ll look to have this replaced.

This is really helpful to know in case I start seeing the same on the other windows in the room. Hopefully this is the only one.

You’ve saved me a lot of time trying to remove it!

Thank you so much for your insight everyone!