Yeah, I know. One of the moving sprites is the background, but the enemies need to move as well.
IF I were you I'd set a timer variable set to 10 centi-secounds, or something better (depending on the speed you want) and then in the block you type:
x = x - #
SPRITE 2 (backround) x y data
to make the background move, and you'd need the code for random enemy spawns.
I would do it this way and then have extra levels on different cards. So that you can still have a MAX of 8 enemies.