Help with corrosion

[amphisbaena]

I bought a sestertius of Domitian last year. The obverse is intact, but the reverse is partially covered in encrustations, and there are also a few spots of active corrosion. The attached photo shows the coin at the time when I bought it. The spots of corrosion are visible, but not very noticeable or 'bad'. Recently, these spots have flared up quite badly, to a bright green. I am worried about this, and I wonder if there is any way to treat the coin without further damaging it. I would appreciate advise in this regard.