Fog or mist is a concentrated suspension of water vapor, and is formed when water condenses into tiny liquid water droplets in the air.
When the USS Enterprise approached the planet K-G, it passed through a "space fog" containing plant spores which fastened to the hull and penetrated the ship. (TOS comic: "The Planet of No Return")
The planet Dantus III was perpetually shrouded in fog. (DS9 novel: The Siege) The planet Tellar had soft, seasonless mists. (TOS novel: Prime Directive)