Animate the mask in a Sprite Mask

Hello, Is there a way to animate the mask in a Sprite Mask? For example, let’s say you have a square as a mask and you want to stretch it into a rectangle to reveal more of the sprite it is masking.

I figured this out, I can animate a spritemask, I was under the impression sprites had to be a child of the mask (so transformations would mess with the children), but this isn’t the case.