Alot of cleanup crew will naturally sent loose particles floating around the tank, I often watch as hermits will send little particals floating off into the current as they sift through the sand. However you may also be getting some detritus buildup. You can take a powerhead and some tubing that will fit the output of the powerhead and use it to blow the rocks off, you would be suprised at what can come out of the pours of the rocks.
Cleaning the filter might not hurt either.
Thomas