Asterina. Personally I take the wait and watch approach with Asterina. If I notice Zoanthids missing and Asterina near by the Zoanthid colony I rehome the Asterina. Some types of Asterina can become invasive and it really up to you if want to intervene.
I know Indo-Pacific Sea Farms sells them but it's a two day shipment from Hawaii. I thought they might make for a cleanup crew. IPSF also sells micro-brittle stars. AFAIK they're reef-safe and excellent scavengers. I thought if the Asterinas got too prolific just get a Harlequin shrimp.