Hi Otlichnik,
I thought I replied to this, apologies.
Thank you for your answer.
From what I read in this
forum and other resources, that was
bronze disease.
I removed it (quite easily, that was not a
good sign) all of it, from what I saw using the magnifying
glass.
The coin is strange, as on the edges it simply exfoliated (never seen this before) but it has an attractive olive-ish
patina.
The
obverse is in a better shape than might seem.
Will inspect it in the future to see if the problem re-appears.