Stain on RGB LED Screen

Looks like I just can't live without causing myself some trouble

I could get a sweet deal for an RGB Led screen to upgrade from my double CCFL one, and I must say the change was impressive, the colors looks like someone's hand is grabbing your head, smashing it on the screen and screaming "Look, this is red !".
Unfortunately I tried to clean it from all the stuff it was stained with after the long trip to my house, using some window cleaner (yeah I'm dumb...), and after that a small, blueish stain appaered on the lower bottom half of the screen.
You can't really see it unless it's on a white or any other bright color background, and only if you look it from an angle, you can't see it if you look straight at the spot.

At first i thought I had damaged to panel backlight or pixel by putting too much pressure, but the spot is so low on the screen I can't even push that part to directly touch the panel, as for the backlight the spot can be still seen on a black background if you look it from some extreme angles.
I think it's either the AG coating that got damaged by the cleaner, though I've tried to "damage" the screen by using the same product on the corners but it did nothing, or some kind of oil/glue that was in the package and got spalmed there while cleaning and now won't go away.
Any suggestions ?

 

Alcohol at 90° find it in pharmacy.

 

I recommend at least 99% anhydrous alcohol (water in alcohol can compromise cleaning) and you can pick that up at any local computer parts store.