Since your previous efforts proved not effective, I would suggest the following solution.
1/ Thiourea [white crystals with formula CS(NH₂)₂] - 8 g
2/ Water 100 ml
3/ Phosphoric (also called Orthophosphoric) acid 2 g
4/ Alcohol 6 ml
Components 1 and 3 could be purchased in specialized shops selling chemistry materials, maybe somewhere else also but where - I don't know. No. 4 could be replaced with simple vodka
The solution should be prepared in this order. If some crystals are not dissolved, that's not a problem, leave them at the bottom.
The solution is not very dangerous to handle, but anyway I would recommend wearing surgical gloves.
I am 99% sure that a dip from a few hours to a day in this solution and a cotton bud will clean your coin.
Don't forget to finish cleaning with good rinsing and drying the coin.
By the way, the same solution is good for cleaning rust from iron coins and conserving them against future corrosion. Naturally, you don't clean silver and iron coins in the same glass.