Seeing imprint marks on screen?

[MiniD3]

Hi guys
Just noticed this ever so slight marking of the keys on the screen,
I can only see this when the MBP is turned off and the screen is dark
I guess that "original" tissue paper that came with the MBP was to stop that

Is this something to worry about?
Should I be putting something between the screen and keyboard

Late Edit:
I used some distilled water and soft tissue after brushing away dust,
The screen is now clear

Is this just a gathering of dust etc around the keys that leave the residue on the screen?

 

[tubeexperience]

just some residue

 

[MiniD3]

Much appreciated,
Will try and keep the dust down, 13000 of travel in the last 3 months in remote areas most have done it

 

[Newtons Apple]

Oil,from finger on the keyboard will leave marks, especially if you laptop has weight on it, like in a backpack.

 

[MiniD3]

I guess 3 mths in the tropics would have been the culprit