Test for nitrates and phosphates. Steady weekly water changes with RO/DI saltwater mix . When doing water change , take out any rock you can easily remove and scrub it in the old water before putting back into the display. Use a toothbrush to scrub any rock in the aquarium and use a turkey baster to blast the rocks free of any debris . Increase the flow inside the display to keep things suspended , net out what you can and let the mechanical filter do the rest. After the water looks clear and things settle , replace filter sock with a clean one and adjust flow to normal speeds . Repeat the rock scrubbing every couple of days to get it under control . Add a clean up crew including snails and crabs ( if you are willing to deal with the trouble crabs can bring. )