Leica M9 viewfinder damage: what is this?

[jmonat7]

So I am in the market for an M9 with an updated sensor and got a good offer on one. However, there appears to be some type of viewfinder damage, possibly coating loss? The owner states it does not affect the viewfinder or functionality but I have never seen this before. Does anyone have any idea as to what this could be?

https://imgur.com/a/R9S5fFf
https://imgur.com/a/GwzuKxh

 

[Huss]

Looks like some kind of liquid got in there.