Assuming your cloud is white you are probably having a bacteria bloom in the tank. This happens for several reasons including but not limited to old food in the water that you didnt get cleaned out, excessive feeding, new fish, fish in a growth spurt, dead fish or plants, etc.

Normally if you see this it normally clears by itself in a few days. You should check Ammonia, Nitrite, and Nitrate in the tank to make sure they are in normal ranges. If they are not doing some water changes till they are in the right areas is suggested. Also check the tank well to be sure there is no rotting material in the tank. Totally cleaning the tank should be avoided if possible due to this removing alot of the beneficial bacteria and will probably continue the problem.