Mood Face is an application for the PearPad used in the episode, iOMG. Brad and Freddie invented it together as their end-of-semester project. It is used to read people's mood by analyzing the blank, almost stoic expression on their faces.
In the episode, iOMG, Sam uncharacteristically offers to help with the project as one of the test subjects. Once tested, the "Mood Face" applications reveals by the blank stare on Sam's face that her mood is "in love."