They might eat them but since it is not their primary food source I wouldn't rely on them as a control method.
You may need 2, buy them from LFS where you can trade them back. Once all asterina gone it's too much work to keep harlequin.
One harlequin will wipe them out quickly. You might need to return the harlequin due to them eating the asterinas quickly. We got one and I'm really surprised how quickly it took down our infestation. We almost got 2 and glad we didn't. We easily had 70 plus on the back wall in the mornings.
We've grown attached to the little guy so we might start feeding him or rehome him. They are really cool though.
How do get them out of a reef tank once they take care of the stars? I'm finally clearing up hair algea and I'm seeing huge amounts of coraline being eaten.