+1 to cloak. Anecdotally, only about 10% of asterinas are truly harmful (typically zoa eaters), so most folks just keep an eye on them to see if they're hurting anything. If not, they're kinda cool looking. I just bought a harlequin b/c I've now got too many of the little buggers in my tank at the moment. If you do get a harlequin, keep in mind that you'll need to find it a new home once the asterinas are gone unless you have a fairly large tank that can replace asterinas as fast as the harlequin is eating them.