Lonnie, when I run into that problem down here on a new clients system here is what I do and it has always worked well.
Carefully siphon the slime off the top of the sand bed and the loose stuff on the rocks, then use a gravel vac to pull dirt out of the bottom of the sand bed. You can slightly kink the hose to slow the flow so you don't siphon your sand bed into the bucket. You will be amazed at the dark cloud that will come out each time you plunge down into the bed. This is where your problem starts. Doing this will pull filthy water straight out without letting it mix in with the rest of the tank water.
Set up 2 tubs with clean salt water and scrub / rinse the rocks that are covered in slime.
Do a very large water change 40%-50%.
Add a large amount of good carbon and Phosban to the sump.
Add power heads or re-adjust return nozzles to move the water in a different pattern.
Be ready to do at least another 20% water change in 1 week.
Be sure to check around in the sump and filter pad or bags for remnants of the slime. Also do a good scrub job on your overflow. Remove the pipe that brings the water out of your overflow so it can drain to the bottom, stir it as it drains so any sediment will be removed.
I have always had 100 % good luck by doing the above steps.
John