Suppose you have two layers then one of them you put in Parallax 0.0 this is the Layer where the HUD or UI and the Player goes. In the other in which you want the Parallax effect, you must decide what type of Parallax you want horizontal, vertical or both.
In case that Horizontal only affects X then you place the Parallax in 50,100, when moving the player with a ScrollTo for example the Parallax that affects the Background will move only in X.
If you want it to be X and Y places the Parallax at 50.50 for example