A Stock Phrase for being on camera. It could be a news camera, or it could be a surveillance camera, if it's too late to get out of its line of sight. There doesn't have to be any guarantee that "mom" is watching. Chances are, if a video camera is pointed at someone, he (and it is usually only one person who does it) will say, "Hi, Mom!"
Compare Confused Bystander Interview, I'm Going to Disney World. [tvtropes.org]