Powdered Sugar

Hard candy is often sprinkled with powdered sugar (or artificial powdered sugar) so that it doesn't stick together.  This is true of even individually wrapped ones because the maker doesn't want them sticking together before they get wrapped, whether in a kitchen or in a factory.  

I know this because I make hard candy every Christmas.  

Anyway, that's what you're seeing on the outside of a cough drop, either powdered sugar or a powdered sugar substitute.