What do you do when you're approaching a green traffic light?
When the traffic light comes into view and it's green, it was likely green for a long time, unless you also observe cars just starting to get going. If you see the traffic light is green in the distance we call this green light "stale", meaning it could turn yellow any second. A traffic light that just turned green, we call this green light "fresh", meaning it just turned green and it won't be turning yellow for a little while. Of course these times between from when it turns green then back to yellow and red again varies on the intersection.
To answer the question when you're approaching a green traffic light you should prepare to stop as that green light is "stale". You don't necessarily have to slow down just get ready to hit the brake.