Is it flakey? And when you scrape it with your fingernail does it come off? If so then you have bronze disease. Let me know and I'll give you the link to a post I did on combating a coin in bad shape with BD.
I also would say BD. Contact @BadThad and get a large bottle of his product to combat this stuff. I have seen it before on Indo-scythian silver coins. Sometimes they had bad mixtures or yours could be a fourree. If you do not treat it, it will eventually just dissolve. Also, it can contaminate other bronzes. DO NOT store this near other coins, and always wash your hands after touching this before you touch another coin.
