This is what Toyota says
The eco-indicator can help you get more miles out of each tank of gas by coaching you to drive more efficiently.
If you don't see it in the Multi Information Display, press the Display (or DISP) button on the steering wheel repeatedly until you do.
The closer the indicator bar is to the left, the better your fuel economy is at that moment.
When you use a light touch on the accelerator, the bar will stay in the eco range and the green eco-indicator will appear in the instrument display.
When you step hard on the accelerator, the indicator will disappear and the bar will stretch beyond the eco zone and start flashing. That just means it's most likely that you're using more fuel than you need to.