The surface of the mirror is cooler than the surroundings. When the water vapour comes into contact with the surface of the mirror, they condense.
It will do this anywhere where the condensation comes into contact with a surface or thing that is colder than it is.
Clouds or fog or dew.
Nothing, water particles are the condensed for of water vapour, or steam. you can freeze water particles to get to ice, but condensing water particles can not be done, they are already in the most...
Rain