As the test warms during daytime/sun the suspended water becomes dissolved and the biodiesel layer is clear. As it cools the bio can't hold as much water so some becomes suspended and it clouds again.
If you put it in the fridge overnight it would encourage more water to become suspended, if this water increased to a certain level it would become free water and drop into the bottom layer. When it was removed from the fridge it would probably be noted that it would stay clear at a lower temperature. I would also think the opposite is true in that on a warm day some water from the bottom layer would migrate to the bio, making it go cloudy when cooled at a slightly higher temperature. I've not tried the above, just my mutterings!