I have a problem with my viewfinder glass surface so I'm not sure is this greasiness or is glass coating damaged and how to clean or fix that?
It looks like coating damage to me, but perhaps a quick test which won't cause any further damage would be...
Get a soft micro-fibre cloth - one of the 'fluffy' ones you can find in any supermarket, not the 'smooth' ones generally used for cleaning household glass/mirrors etc. The 'fluffy' ones are more gentle & will not scratch even plastic.
The viewfinder looks to be recessed too far to be able to reach with a finger, so roll or fold it to generate a 'point', avoiding any of the edge stitching.
You could wrap a couple of layers round the end of a paper-stalked cotton-bud instead.
Breathe on the viewfinder or the 'point' you just made in the cloth until it is barely moistened.
Gently wipe the surface of the viewfinder.
If it's grease or dirt you will see at least some slight cleaning; or even just 'blurring' of the markings if it just shifts the grease slightly.
If the pattern doesn't change at all, then you've pretty much confirmed it's damage to the coating.
btw, you can use the same technique [without requiring the 'point'] on the rear screen, which is filthy;)
It could be residue or coating abrasions from prior cleaning attempts. The procedure suggested by Tetsujin should help you determine which you are dealing with.
Coating abrasions are best ignored because they normally do not affect lens or viewfinder use. Trying to "clean" them away will just make them worse. They often have a fine, scratch-like pattern surrounding them.
Oil or other residues may be removed with a solvent intended for cleaning lenses. You may use a lens cleaning cloth or tissue wrapped around a cotton swab to access the viewfinder lens
Take care not to clean too often or too aggressively lest you create abrasions where there were none. Use a blower to remove debris as needed. Avoid wiping lenses unnecessarily.