Sorry I misled you... It isn't working! Was just pressing enter , and not testing the movie. But it should work!
That blue mask of your's seems to be the problem!
Just re-do it!
Trash the layer on which the blue mask is on.
Insert a new layer. Select frame 10 and press F6 to insert a keyframe. Now select the first frame in this layer.
Select the oval(circle) tool and hit the stroke box, the one over the bucket. Click the dialgonal red line icon to block any outline being drawn. Now draw your circle - color is of no importance, just make it different than the background so you can see it... - so that is covers your "green spider", and center the circle over it.
With the first frame still selected, under Insert select Create motion tween, and under Modify->Transform->Scale and Rotate, set the scale to 10%. Turn your layer into a mask and test your movie. And yes you can keep your white background.
You could select frame 9 and press F6. Then select frame 10 and press Shift F5 until you get rid of all frames after frame 9.
If you now want to prolong your mask's motion, just select frame 1 and pres F5 adding a frame each time.
Once again sorry for my first assuption that it was working!
Regards,
ldnewbie