The Acropora eating flatworm is a predator of Acropora spp (and is much different than the red flatworm/Convolutriloba retrogemma). It can cause tissue loss and eventual loss of entire Acropora colonies. The flatworms lay eggs, and can lead to large populations that become devastating. Levamisole is one treatment option for the flatworms in a dip form (not in tank) with subsequent dips at the right time for newly hatched flatworms. A search should help find the latest treatment methods, and dosages of medications.

This picture shows two flatworms on a Acropora spp. Sometimes the flatworms are not so obvious on the coral, and they can blend in so well, they are nearly invisible until they move.