This animal is part of the Class Ophiuroidea. Species are commonly referred to as brittle or serpent stars, usually depending on their appearance (obvious arm spines get the name brittle, while smooth arms get serpent). This is a distinction that is made within the hobby only. There is no scientific distinction made between them, they are all brittle stars. Most small species are reef safe scavengers and valuable additions to the live sand community. The linked picture above by John Newton is the ideal species for a reef tank and is offered by many online mail order companies that offer live sand activator or detritivore kits.
Small Brittle Star
Scientific Name: Ophiuroidea
Identification: These small starfish resemble large brittle stars but they are in fact a different species
Treatment: Mostly harmless
Prevention: Quarantine rock and sand before adding to your aquarium