The easiest way would be to make a sprite object and clone it as many times as you need cloud spawners. Now place one of these "cloud spawner" cubes at each cloud spawn location. Then either use Every X seconds or Every X seconds with Random(0-#) as seconds to spawn clouds.
On your cloud object itself use the bullet behavior to make it move right and a fade behavior and set the fade to begin when spawned via action and set the fade timer to be whatever lifespan you want for the clouds with a destroy action after the fade action.
If your game doesn't scroll to right you could also put a rectangle sprite just off screen that is the height of the screen and use an On-overlapping this "cloud killer" condition on cloud to destroy clouds after they leave screen.
Also if your game does scroll right you could put the scroll-to behavior on all your "cloud spawners" and adjust the offset so that they stay off screen behind the player/camera but follow along behind spawning their clouds.