Horn silver is silver chloride. So long as the silver is relatively pure (as it should be for an early Sassanian drachm) and the coin is kept away from sources of chloride in an acidic or oxidizing environment (eg. being buried again in salty, acidic soil) it should not spread further.
"Bronze disease" on bronze coins (or on debased silver coins) can spread because it is mostly composed of hydroxides; it can suck everything it needs to spread out of moist air.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis