Hi...
Yes, we can create a mask that is independent of all other layers.. That mask layer is called 'Shape Layer'.
Without selecting any layer in your composition, click on the Pen tool icon in tool bar, then draw whatever shape
you want with that pen tool. it displayed on timeline as a Shape Layer1. You can animate it independent of all other layers with its Path attribute(Contents->Shape1->Path1->Path).
In this process you will get a filled shape layer, to avoid it click on "Fill" in your tool bar (click on only "Fill" word, not the fill color). Then you will get 'Fill Options' dialogue in that select 'Solid Color' as None, click ok. Now you will get path type mask , instead of filled one...
hope you get it...
Find my AE Scripts here..
manohar