Urgent Ident! What is this? Is it dangerous?

Some sort of Sea Star is breeding in my tank.



I was too nervous to take out the bigger ones, like 5 inches in diameter, but found this baby on the glass.



Please identify it ASAP- theres 4 I can see, so there must be dozens I can't.



Nanocube w. newly installed Radion, MP10, Nitrate Ammonia Nitrite 0,