Are you talking about the "Cloud-tank" shot from ID4 right?

Well, that was done by pouring a mix of milk and fullers-earth into a tank of water, and shot with the frame-rate changed to emphisize a sense of scale.

In your case, I would suggest fluids, as it is a LOT less messy hehe..

Check out the fluid presets in your visor for something cloudy to start from, and add a dynamic dummy-object to fly through it, and use that as a starter for your composite.

Does that help a little?

PS, welcome to SM!