I was thinking that maybe if you had calupera or some others that are known to go s-e-x-u-a-l, they will release their spores into the water and it will be very cloudy, almost milky. Reason I asked how your calcium was, is because if your calcium is abnormally high you could have some precipitate. Which, again, will be a white cloudy tank.
Can you think of anything out of the ordinary that you did? When did you last do a water change?