How to get rid of screen "stain"

I don't know how it got there, maybe while movbing thr iMac, but now there's a sort of blue stain that looks like a cloud in the middle of the screen. It's like when you touch an LCD screen with your finger, it will produce a mark and diseapear after a few seconds. But this stain on my screen is permanent.

Any tips ?

Thank you. I guess I'l have to live with it then.

A 2 inches blue cloud ain't that bad after all.

It's not as bad as the time my daughter "painted" my 22 inches screen with a magnet...

Had to buy a machine to take care of this one