Is your substrate sand? Looks like it from the pictures. If so, I'm guessing it's sand--sand dust. Is there a fine layer of white on other things, like the decor? This sand dust would settle on things and float around if the sand was not washed before adding it to the tank.
A newbie friend had this sand problem when starting a small tank. In five weeks, with two filters, this sandy dust would come and go, sometimes clouding the tank so much she feared for the inhabitant's life. It just would not settle permanently. She didn't know the sand should've been washed first. Ultimately she dissembled everything, washed the sand, and put it all back together again. Problem was solved.