They'll spread like crazy. I have tons of them and they don't create any issues. I actually got to see a spawning event a few nights ago. I saw at least a couple dozen climb up to the highest nearby spot and stand up on all 5 (sometimes 6) legs and squirt out their reproductive juices.
only a select few asterinas will eat corals..especially acroporas. However at the aquaculture facility i work at i have seen the speckled ones as well as the tan/off whiteish ones and haven't really seen them pose a large problem. At least not as large of a problem as some other pests like AEFW's or montipora eating nudibraches. However we still deal with pests as a potential danger and try to rid the systems of all undesireable pests. The acro eating asterinas we deal with by manual removal and we also have harlequin shrimp trained specifically on asterinas. The thing with the asterina eating stars that i have noticed in my own experience is that they dont only eat coral and they are also ditritivores. This is why they arent as large of a threat as most of the other pests you would find in a reef tank. Im sure others have different opinions on them but i am just speaking from my experience.