The only problem I ever had with sallys being aggresive was towards other sallys. I never had much success keeping them long term though, six to eight months. Sallys mostly work the rocks, I don't recall seeing them spend much time on the sand unless there was a chunk of food. I have a couple serpent stars and a brittle star that do a good job of cleaning up detritus that I have had for a few years. The easiest thing to do is just vacuum up portions of the sandbed and clean it yourself. Once you get your nutrient levels under control, the sand should stay clean.
Todd