I was having this problem at about the 5-6 month range for my tank. I threw a sand sifting star in there and the sand has looked pristine since. I know that he is not eating the algae (pods is his diet of choice I believe), but he does a really good job of keeping it stirred up (with help from a couple dozen nassarius) so I never see the algae at least. I had tried a queen conch before the star and never saw the results I was hoping for.