What are you prepared to feed the harlequins starfish if/when they erradicate these Asterinids?
Some Asterina are sometimes reported to eat coralline algae, but in most tanks, this should not be an issue. Coralline grows fast. The same complaint is made against many urchins, and many people avoid them for this reason. But many people will argue that the coralline regrows quite rapidly anyway. As far as polyps are concerned, that is difficult to say. They will often show up on glass and LR....wandering around...and by default end up in the general area of corals. Have you seen them on the polyps? There may be some out there that will eat polyps, SPS corals, etc, but IMO it is not the common form. Indeed, I have loads of these things reproducing in my nano tanks, but have no SPS corals, which they are often condemned for eating. There is a lot of debate about this issue. Many would argue that the stars may be starving in some systems, and may thus be drawn to eat things they would not normally eat. Others may say that this is an individual issue, and not a species issue. Finally, many would say that the stars are drawn to corals that are weak, stressed or dying due to other issues...something that the stars can most certainly 'smell' and will act on long before we recognize that the corals are ill. If they are causing trouble in your system, then by all means remove them. But I am not sure a harlequin is the best solution to that.