The only thing that comes to mind it to buy a piece of white plastic moulding, the ones that they sell for the edges of those plastic shower surround panels. They have one where the profile looks like a small "j". I would mastic this into place so that the thin edge was just over the edge of the tile and the boxed end was showing. Or, I would probably dress up the whole edge with some other piece of trim. If the fixtures in your bathroom are silver, then maybe a chrome "L" affixed over that edge, or if it lines up with the edge of the tub, then cover it by putting in a shower door.