... is it a piece or a fusion? There is no piece in there, I'm fairly sure of that, but I can't recall if there is a fusion in there. But it might be one, many of the fusions are restricted to different times in the game.
I don't have a save file for the phases after you beat Vaati, but I don't think you can get into the library after Vaati, if I recall correctly. But the only time you are needed to get there is to get the flippers anyway, and there is nothing else in there.