If the target creature is an illegal target by the time Glacial Grasp tries to resolve, the spell doesn’t resolve. No player mills or draws any cards, and the creature won’t be stopped from untapping. If the target is legal but Glacial Grasp doesn’t tap it (most likely because it’s already tapped), the rest of Glacial Grasp’s effects still happen: that creature won’t untap during its controller’s next untap step, that player mills two cards, and you draw a card.