There are some chitons that stay in the sand and graze below the sandline. I've seen these in another reefer's tank. The same reefer told me that he had dealt with a different variety of chitons that were grazing all his coraline algae up on the rock by taking apart his tank to remove them. So, I think different chitons graze different things and not all of them are necessarily desirable.
I think Edge's suggestion is the best way to deal with algae growing on the glass at the sandline. If you try to deal with it with a magnet, you wind up scratching the tank. Periodically dealing with it with a razor blade has been my method on a glass tank, but this is no good for acrylic.
As far as areas of mulm on the sandbed, I get this in the back eddy areas of my tank. Macro sand grazers that turn over the entire top layer of sand might help to keep these areas from occuring, but as some of these are grazers of micro sand infauna, I would be cautious.
Cyanobacteria colonies growing on the sand are a different beast. Some types I associated with high tank nutrients and are dealt with by controlling nutrients. Other types I associate with phosphate recycling from the sandbed and control is more subtle. The thick matt types can be siphoned out. Otherwise, knowledge, experience, and patience is the long-term solution to prevent obvious colonies of cyano, IMO.
