These guys occupy a similar niche to bristle worms, but they will not hurt you! The arm length of these guys usually will never reach more than ~1.25”.  They hang out in the rocks and not often seen except at night or when food is present. They will help clean up uneaten fish food in your tank before it has a chance to decay. They spawn in the water column and will increase in numbers in most tanks. They are very small. Their arms may stretch out as wide as a quarter when full grown.  Some people may call these, "Micro Brittle Stars" which is fine.  Other people may call them a "Baby star fish" which isn't a good as it may leave the impression that they will grow to be much larger.  

Please note coral in images not included with this item.