Ever since Finn got his arm back in the episode "Breezy", he still has the Grass Sword but he doesn't control it. It controls it's self! If there is a big situation, the Grass Sword activates and Finn uses it to fight with it like the episodes, The Comet, Crossover, and I am a Sword. The Comet and Crossover episodes the Grass Sword did control it self in those episodes but for I am a Sword, I'm not sure and I think I know why. Theory #1: Finn controlled it like what he used to do when it was around his wrist. Theory #2: The Sword did control it's self and Finn noticed it by looking at his arm because his arm was shaking while activating. Those are my theory's about the grass sword in the episode, I am a Sword.