Here is what Dr. Ron had to say in Marine Invertebrates "Obligate scavengers on meaty foods. Will not eat detritus or attack healthy animals." , and "Specialized to eat carrion, not live flesh, they will rapidly consume leftover food."
They will eat leftover fresh food not fish poo, so this needs to be taken into consideration when feeding you're fish you should feed extra to support nassarius snails. The first Nassarius I had slowly disappeared within a few months because I was too conservative about feeding. Nassarius are beneficial because they help to stir the upper layers of you SB, and they will clean up after feeding you do however need to be careful how many you add unless you are prepared to feed them as well.