Those emerald lights are there to outline the periphery of the step on which you are about to hop or hop off. The majority of accidents on escalators occur when a passenger missteps upon entering or exiting the escalator. The lights, which are located just below the first step of ascending stairs (and the last step on descending stairs), are there to show the way for the unproficient escalator passenger.

Escalator lights are green for the same reason that traffic lights are green: Green is among the most visually arresting colors.

From: When do Fish Sleep?
by: David Feldman