WS2812-and-similar individually addressable LEDs are controlled by the data sent by the microcontroller (on that third wire, which is apparently green in your case - the colors aren't standardized). If you do not send any data, they won't turn on - you haven't told them to! They're obediently waiting for data to be sent down that dangling wire (it would be really bad if they did turn on of their own accord, because then you'd always need a power supply that could handle all of the LEDs being on at full power; it's quite common to use a smaller power supply if you know your code won't ever turn them all on at once)