It may also be your tap water, have you filled a glass of water lately? I've noticed that my tap water has been cloudy lately, (I believe someone on this forum said it's just oxygen bubbles, not sure though. I remember the cloudiness is harmless though
) and when I do a water change it clouds the tank.
If not though, I agree with the others that it's a bacterial bloom. Although, what filter media is in the filter? You may want to look at adding a polishing pad (I use Sera Filter Wool, cut to fit) to help remove particulate matter.