There's a number of ways to have the clouds infinitely scroll.
1. You can make them tiled background objects, and use image offset (could take up a good bit of VRAM though)
2. Use ScrollXleft or ScrollxRight and reposition the clouds when they go off-screen. If the cloud's hotspot is in the middle, then something like this:
+Cloud.x < ScrollXLeft-(cloud.width/2)
-Set Cloud position to: ScrollXRight+random(n), 0+random(windowheight)
3. Use the wrap behavior (that might only work well with single-screen levels or small layouts)