This is the Order of Leopold - one of the highest orders in Belgium named after King Leopold the first. It is awarded based on length of service rather than valour (not to take anything away from your grandfather!) and can either be a civilian, military or maritime division.

What puzzles me is that in most examples of this medal there are crossed swords between the top of the cross and the ribbon, which don't seem to be here? Maybe they've been knocked off? Someone might be able to enlighten you further :)