making a alpha mask

Hello. I am new to flash and have a specific goal I want to achieve.

I want to create a mask where all of the stage is about 30% transparent white mask and the moving object is of course 100% transparent. The end effect will be a white haze (the alpha'ed mask) over an image with an oval of full transparency moving around the image.

If not a mask, is there another way?

Thanks
JamesI