Wow! You guys did a really nice job on this! I love how you both created depth with the back and foreground; I also like how the robot-alien-thingie seems to have appeared out of no where from the fog (I personally always found fog/mist hard to create).
There's this pack of concept art brushes that have some of the best cloud brushes. They really help with painting in atmosphere. I'll send you a link when I figure out where you can download them.
Awesome work you guys!