I've thought of a doctor who alien!

It looks like a cloud but it's actually a lave living in earths atmosphere, taking nutrients out of the air.
The alien that this lave belongs too put their children in the atmosphere of different planets and when they're ready to hatch they fall to the surface of the planet and hatch and eat everything they can in a 20 mile radius. They then fly off into space in search of the nearest flock of their species.