Of course he can change the ice's direction. Read David's post. What he cannot do is make it appear away from his body without a connection, so the source will always be him. That's why having ice snake around to flank Izuku is a slower attack and uses more ice (and most likely focus) than his straightforward attack, which was already extremely effective because it forced Izuku to use up his body parts. Shoto just didn't think Izuku can cheat by reusing fingers.