I would say there is two possible things this could be - bacterial bloom or calcium precipitation. what is your mag level in the tank.
There are many links on google about bacterial blooms in aquariums. Here is one:
http://www.algone.com/cloudywater.php
if your mag level is low and your calcium is that high, there is a extremely good possibility that you are getting calcium precipitation. If you are adding anything for calcium you need to stop till it drops to normal levels. Mag levels should be around 1300, i shoot for 1350