IMO It's just a black nassarius snail nothing to worry about. They eat food were ever they can find it, sand or rock(sand preferably). I bet if you gave your rocks a few puffs with a turkey baster it would produce a few clouds of detritus.
That is what it is eating.
I don't know how old your tank is, but if your sandbed is relatively new, there may not be enough detritus, broken down food, and other microfauna it eats there.
Here are a few places that sell the black ones. I have black ones in my tank, no issues.