Grealish is Grealish. He holds the ball up brilliantly and, by backing defenders deeper and deeper, sets up space for a midfielder to burst through for a shot - Garner usually. He needs a better overlapping full back next to him, and he needs someone with Dewsbury Hall's skill set to link up with. Perhaps the problem is that Garner and KDH - while both great players - aren't City quality players, neither is Myko - again, a solid enough left back but not Grealish levels of ability.
Moyes has told Jack to do what he wants once he has the ball, but even a player as good as Grealish still needs a certain amount of coaching, even if its just how to work intelligently with the level of players around him. He definitely looks like he overplays at times, but that might be because options aren't open to him, or it might be that he's become hyper-aware of his importance to the team. Plus, he's desperate to get back into the England squad, so that might lead him to try almost too hard.