Need help making the background move (.fla included)

Hi, im making a game and I want to make the background move separtely from the level (so you can’t walk on it). But I still cannot find a way. I have included a .fla to show you what I have done so far.
(basicly I want it so he don’t make contact with the trees)

Here is the download link for the fla file, I need to keep the action script already there the same, or simular, so the game still works the same.
Thankyou to anyone who can fix this.