Eternally floating? Like, scrolling clouds maybe?
The easiest way to make them is to create a SPRITE, and make it look like a CLOUD. Then, apply the WRAP BEHAVIOUR to it, and the BULLET BEHAVIOUR.
Set the BULLET BHEAVIOUR to make the cloud always move at whatever speed you want, and set WRAP to enabled. When the cloud leaves the screen on one side, it will appear back on the other side and keep scrolling.
You can get fancy with it, and make events like