They are save and they reproduce by budding, I believe that's what it's called. Those tiny ones you have around the large one are new ones growing. They will reproduce if the conditions are right but I wouldn't expect them to overtake the tank. They are a safe coral to have, not a very aggressive coral and are pretty low maintenance. They should get what they need from your lights and from water changes, as well as what they can get from you feeding the tank.