Hi...I just finished clearing a terrible black/green-then-white cloudy mess (late opening) with the help of everyone here (read my story, if you'd like under "Calcium Precipitate Cloudiness...").
We changed the sand in our filter, which was 5 years old, about half way through the process and I really feel it did a lot toward clearing the problem(s).
But, you really do need to get a good test kit, know your CYA so that you can shock to the appropriate level and stick with it. New sand won't do it all, though it is worth the effort. My husband and I were horrified by the small opening in the top of our filter, but after about 30 minutes of careful scooping, we were able to turn it on its side and speed things up (350 lb. filter).
Good Luck!