Does the plumber fix this or my tile guy?

My plumber installed our steam unit, however we have not yet used it or the shower as we are concerned that water will get behind the piece where the steam comes out of. When we breached it to the plumber he said he can't make it flush with the wall without causing damage. He said to ask our tile guy what to do. The tile guy believes it needs to be flush with the wall.. what should we do here?