No, the pasteurized whites aren't raw per se; they are heat-treated whites, and they are heat-treated to help kill off harmful bacteria. Of course, if you leave any moist object out on the counter for a period, it can grow mold. Or if you cross-contaminate it with something else not in the egg whites, you might get into trouble. But, if you do neither of those things, you are simply setting the icing out to dry, which it does quite quickly. As the icing dries, the conditions under which bacteria can grow are greatly diminished, especially if there was little to no bacteria in the eggs to begin with (and you didn't cross-contaminate).