How to use mask with animation loop?

Code used to attempt this so far:

I want to find a way to be able to slightly edit a mask on every loop to create an animated effect. Such as on each loop increasing the Y coordinate of where the ellipse is drawn over the mask to make it look like a circle is moving up and down, while maintaining the graphical mask.

So far, the code seems to only draw the mask once. What am I doing wrong?

Any suggestions?