How to deal with Stonewall Attack 'counter'? I've been trying out the Stonewall Attack opening and have had quite a lot of success with it. My main problem arises when my opposition doesn't castle on the king side and instead moves their king towards the queen side. It tends to be lower ranked players who do this, but I always find the games a lot tougher than when they castle their king, as I have good control of that side of the board.

My question is, what would your advice be on proceeding if this occurs? The game turns really messy, and I feel like I have less control. I am quite a new player so please bear that in mind :D.
No opening approach is equally well suited to all responses.  The Stonewall does often allow for an active king bishop, an active knight on e5, an open f-file, and the promise of a g-pawn advance against a castled King.  On the other hand, it means a diminished role for the queen bishop and (if things don't work out according to plan) the queen knight, and it gives White a backward e-pawn and some central holes.  White's center can easily become weak without a lot of piece protection there.  You'll want to be mindful of these tradeoffs when using this "telegraphed" opening.

I first learned about the Stonewall from a book by prolific author Fred Reinfeld called How to Think Ahead in Chess.  He devoted almost half of this small book to this opening, basically endorsing it wholeheartedly for the beginning player and failing to discuss its drawbacks.  He even encouraged the reader to stick as closely as possible to a specific  move order and a predetermined set of opening- and middle-game objectives.  An extraordinarily bad set of advice, on later reflection.

EDIT: You asked for advice about adapting to a Black king move to the queenside.  You might want to try to make the most of your f-file strength, going after the f-pawn and penetrating the Black position on f7.  If black creates good counterplay with a c-advance and good hits against your center, you may be able to avoid serious problems by exchanging pieces and setting up a chance to liquidate your e-pawn before it becomes an endgame liability.