Mechanical Removal
The Rust Brown Flatworm does not attach itself to the coral and is easily removed by using a small (1/4" air line) siphon. Simply just start a siphon, and then gently vacuum the flatworms from the surface of the corals, being careful not to contact the coral's surface with the tube.
Another method is to use a brief freshwater dip or bath. Simply submerge the coral in a container of dechlorinated freshwater for 5 to 10 seconds and shake the coral. Flatworms are very sensitive to changes in salinity and will lose their grip on the coral and fall to the bottom of the container in a short period of time. Before dipping the coral, balance the salinity and pH to match that of the saltwater the coral came from.
Natural Method
Flatworms have a number of natural predators, including the Sixline Wrasse (Pseudocheilinus hexataenia), the Yellow Wrasse and the Spotted Mandarin. The biggest drawback to ridding your tank of flatworms with this method is that the fish will not consume every flatworm in the tank. The Blue Velvet Nudibranch (Chelidonura varians) is believed to do an excellent job of consuming flatworms. The greatest problem with the little (2") Blue Velvet is that flatworms are its entire diet and once it has rid your tank of all the flatworms, it will slowly starve to death.
Chemical Treatment
There are a number of chemical flatworm products on the market which seem to work fairly well without harming your other tank occupants. Salifert’s Flatworm Exit™ is a widely-used flatworm eradicator and has several good reports.
If you elect to use a chemical treatment, be sure to siphon all of the dead flatworms out of the tank, as they may contain toxins which can be released back into the tank water as the flatworms decay.