Why does moist in the mirror after you blow it?

The moist you are talking about is nothing but the water vapour that one releases, every time one exhales. As already know, when we breathe in, the air that enters gets humidified as it passes through the nasal passages. Hence, the air exhaled, or breathed out, contains considerable percentage of water vapour.

Now, this water vapour exhaled out, initially is at body temperature, i.e., 37.5*C. On coming out through exhalation, the vapour soon cools down and condenses on the nearby mirror glass surface.

Hence, the moist is seen.