Many expressions link two properties together in a relationship similar to parenting, in which one property controls another property. For instance, if you added an expression to a mask's Expansion property, then Pickwhipped the layer's Opacity property, Opacity would control Expansion.
You can always add the expression to the controlled property (in this case Expansion), and then Pickwhip (or indicate by typing) the controller. Then, to cause change over time, animate the controller (in this case Opacity). If you link Expansion and Opacity in this way, then animate Opacity so the layer fades in over time, the mask will expand automatically as the layer is fading.