? Wrong labelled ...onion skin what for here??
Would be more helpful to know if the Animation is with a MC or just a shape.
If the Object is a MC you don't need any onion skin to reduce Alpha.
It's enough to go just on one frame and change one MC in that frame and all other copies of it are changed.
So I don't get the really point of it (obviously you try to make a kind of shade to the moving ball with the 2. layer? So tell it, please)
And maybe I get this completely wrong so I have to agree LegendaryGF: This is quite confusing ....