Thanks for the quick replies!
The close up is rather fuzzy because of the camera; The spot is not deep, but the varnish has been clearly torn off, i can "feel" the bare wood with my thumb. The first picture presents it a bit more clearly than the close up.
Also, it didn't use to be so green, it's turning greener in time. I'm afraid it will get bigger, so i want to fix it now so i won't regret it later. I realize it's a minor thing, but i'm afraid it will get worse in time...