Ilyanassa obsoleta is just a specific species of Ilyanassa sp..
I. obsoleta is one of the few truly omniverous snails. They can eat nearly anything including many types of coral. What they prefer to eat is carrion followed distantly by diatoms but if they get hungry all bets are off on what they are willing to eat. In the typical reef tank there is enough carrion and left over fish food to keep them from getting hungry. These snails are often called Nassarius Snail, they are not. Nassarius Snails are pure carrion consumers and are unable to consume live flesh.