Make sure you know what you're getting into before buying! Harlies are awesome little critters and need some special care. A friend had one that starved to death after clearing a major area of his tank of asterinas, but there will still tons on the other side of the tank. The shrimp just didn't hunt out that far for such small snacks apparently.
So plan on supplementing its feeding with a normal star, and in between big stars it will hunt tiny asterina's all the time and still wipe most of them out. Just don't starve your harlie!