As Burleigh explains:
The one-finger display is the distinctive Muslim gang sign: The index finger points straight up while the thumb wraps underneath and presses against the digital phalange of the middle finger. The remaining fingers are squeezed against the palm in order to highlight the extended forefinger. The extended finger is symbolic of the one-God concept of Muhammad and is understood by all believers to be a symbolic shahada, the Muslim affirmation of faith: There is but one God and Muhammad is his messenger.
Don't worry. I'm sure it doesn't mean anything. That odd hand signal must have been just a weird affectation.
Just move along. Nothing to see here.
Hat tip: BadBlue News.