This is common in new systems (under 6 months old). It is happening in my new 20 gal
right now. It's just the bacteria setting up shop. If you want to speed things along water changes will do that for you. It can also happen in older systems if too much waste accumulates in the gravel. If you have been vacuuming then this is probably not the issue. Also...using activated carbon can help pull those dissolved organics out of the water. That's what the bacteria making the white cloud feed on.
The only other thing that makes white cloudy water is dust from crushed coral, shells, or Sand etc. You would know if that is what it was because it would have happened as soon as you added it in with the water.