Horn silver is silver chloride, AgCl, and usually forms on silver coins in the presence of salt water or in wet ground near an ocean. It is very destructive, but I don't think it keeps eating at the coin after it is stable. If it is anything more than a very light layer, removing the horn silver with an acid or other chemical often takes much of the coin's detail with it, and would harm the coin. Others my have more information on this topic.