You have copper corrosion products on a silver coin. Your coins looks to be solid silver so the copper corrosion is likely from other items that were buried in close proximity to the coin. The pattern of copper also supports this as it looks like the result of water deposits (water in soil that is).
I don't think you can clean it nicely mechanically, as the copper-deposits will be hard and the silver can scratch easily.
If you feel you must clean it you would have to use a chemical (mild acid) that would soften or remove the copper and leave the silver.
Check the "Cleaning silver (not
silvered) coins"
thread in this section for more advice (it is not something I am
good at) or maybe others will add more advice.
Shawn