In reality the green pointer is showing the direction the drone is trying to move in order to avoid an obstacle.
In your video, the part on the water, there is no imminent object in your path, and it becomes active. The key word here is water! The reflection of the water patches and the sun makes the system a bit confused. Try the same exercise closer to mid day and with no water around, I believe it will behave differently. That's my 5 cents.
Sent from my iPad using PhantomPilots mobile app