To share your Mood, tap on the plus button in the center of the tab bar at the bottom of the screen. When the Composer appears, tap on the Mood tab at the bottom of the screen. Here you will see a list of moods which you can select.
Once you've made your selection, tap on it and the Composer will dismiss and you will see your mood update appear in the main feed.
More moods will be added in the future!