mhh this one is a simple, its a Sprite Sheet with some "Clouds" i made in Photoshop, Increasing the Size and change a bit Shape from Frame to Frame, its running like an Animation., which also could be used to make that effect. But with the Particle System you have much more Control over the Sprite Sheet, adjusting Colors, Speeds in Curves, and other stuff.
I Mean its also Achievable with the Animation System, but it would be alot more work. If you wanna make Detail Changes of it
But i also ahve alot of Practice with the Particle System recently, since my Main Project is almost ready for the Steam release, ive been working in the last 2 weeks through all my effects and improved them "DRAMATICALLY" haha