If it is small it is probably Asterina (do a search on the boards). very common. the number of arms may vary in different groups of stars. In Asterina and others, having more than 5 is not at all uncommon.
Nearly all Asterina that end up in the hobby are harmless, but some could be issues. The biggest issue is eating calcareous algae - not typically an issue in most tanks. Rarely someone reports getting a coral eater. However these are very rare all considering the fact that nearly everyone with LR probably has these stars.