The phenomenon you observed is normal. When powering off the projector, the projector will perform Auto Color Calibration in order to automatically keeps the correct white balance for out-of-box color consistency of the projector. The green tint will last around 1 second then the projector will be powered off properly.
However, you can prevent this by using the following methods:
(1) Turn off Auto Color Calibration via the on-screen display (OSD) menu >
Main menu: System > Color Calibration > Auto Color Calibration > Off
(2) Change Light Source Mode via the on-screen display (OSD) menu >
change to Normal or ECO