We all know that sometime during season 1 of Star, Marco breaks his arm and in an attempt to heal it again, Star accidentally gives him a replacement arm-a monster arm. At the end of the episode, when Star is finally able to fix Marco's arm correctly, Monster Arm claims that he is part of Marco now, and he can't get rid of "the virus" Star had created. Marco seemed slightly worried, but after seeing Star's lack of concern, he eased up.
Later on in the season, in the episode Storm the Castle, Star and Marco are seen in a cave. Star is trying to reach a sandwich at the bottom of a hole in the cave. She told Marco to grab one for himself. After looking down at his left arm and having a flashback of Monster Arm, he declines and puts his hand back in his pocket.
In the middle of Storm the Castle, when Marco is being held captive by Toffee in order to lure Star to him, Toffee offers him a sandwich. In fact, it was the same exact sandwich from the beginning of the episode. This is where it gets interesting.
Marco loses his temper and screams at Toffee, saying he doesn't want "your (Toffee's) stupid sandwich!" This is very unusual behavior for Marco. The only time we had seen Marco like this is when Monster Arm had began controlling him and altering his thoughts, making him more angry and sensitive than ever.
I believe that Monster Arm is beginning to make his way back to Marco's mind, and Marco is trying to hide it.
Since Monster Arm is a part of Marco now, he is technically part evil. This means that he is able to control Star's wand, which he wasn't able to do when he was completely human.
In the episode My New Wand, Star's wand is activated not once, but twice when Marco speaks. Both times Marco is stating that he doesn't want something to happen. The fact that the entire episode My New Wand focuses on Star trying to control her wand with her voice, and Marco can seemingly do it without even trying is conformation enough for me that Marco is more powerful than Star.
In season 2, during the episode Star On Wheels, Star loses her wand while she is riding Marco's bike, and he has to get it back to her. But instead of simply picking up her wand and throwing it to her, Marco is very careful not to touch it. Instead, he keeps trying to give Star her entire backpack. I think the reason for this is that if Marco was to hold Stars wand, it would reveal not only that he could control it, but how much more powerful he is than Star, which the writers obviously don't want to show just yet.
Now you may be wondering, why would Marco try to hide this from Star?
I have two reasons for this.
1, Marco doesn't trust Star enough to let her help him. Last time she tried to help him with a broken arm, she only made things worse by creating an evil monstrous virus inside him. After all that, I doubt he would let her try to get Monster Arm out of him.
2, Marco thinks he can control it himself. Marco has been a very independent character ever since the first time we saw him. He most likely thinks that he can overcome the effects Monster Arm has on him. It's obvious now that Marco cares very deeply for Star, and vice versa. He probably doesn't want to worry her, or worse, harm her, so he is trying to keep everything to himself.
Toffee knows Marco is part evil, which is why he was so nice to him in Storm the Castle. He also knew that the sandwich would not only remind him of Monster Arm, but also the fight he and Star had had earlier in the episode. He was trying to provoke Marco to see how far he could make the Monster inside of him, which obviously wasn't very much. That might be the reason why Toffee called him "a disappointment".
Toffee might try to use Marco in the future because he knows Marco is more powerful than Star.
Hopefully we won't ever have to witness "Evil Marco", but I guess only time will tell.
