After researching, it looks like teen may be right. I however also found the a Wrasse may help. Also siphoning them and high water flow. Read here
http://www.wetwebmedia.com/flatwrmfaq3.htm
And here is a write up from Anthony Calfo
Rust Brown Planarians are just one such example of a nuisance organism that needs to be monitored if not controlled. This flatworm is usually 1/8 to ¼” in size (~3-6mm) with a color ranging from orange through red to brown. It is cited in aquarium literature as Convolutriloba retrogemma and considered to be a pest because it can expand in population to plague proportions in a matter of a few short months. Populations get so dense that they occur in crowded mats that literally block necessary sunlight and circulatory functions from the corals that are forced to serve as perches for them. They are said to ingest zooxanthellae from decaying coral tissue and prosper under bright illumination. They dislike strong water flow and are often noticed in the areas of weakest water movement in an infected display. Increased water movement alone in otherwise properly maintained aquariums can sometimes reduce the population of flatworms.
Aggressive protein skimming also helps to control the population of this annoying pest. It has also been demonstrated that low salinity impedes the growth of flatworm populations but at the expense of other desirable invertebrates in the display if applied as a long bath. I once inadvertently dropped the salinity of a badly infected system from 1.023 to 1.017 with a water exchange using water that I thought was salted… but was not. The sharp drop in salinity promptly killed every discernible flatworm in the display, but shocked every Sinularia into expelling zooxanthellae at the same time! The leather corals took months to recover and I would never recommend imposing such shock deliberately. Indeed, short freshwater baths are effective against many planaria, but quite stressful to other desirable coral and invertebrates even on selectively treated rocks.
Natural predation may help but is somewhat unpredictable. Dragonets (Synchiropus species) and Leopard wrasses (Macropharyngodon species) have been used with varying degrees of success. Both fishes are truly in need of care by advanced aquarists. Leopard wrasses and dragonets will survive very well in systems with refugiums generating copious amounts of zooplankton as well. Chelidonura species of sea slugs have also been cited as excellent natural predators, but acquiring an effective species is difficult for many livestock resellers, as collectors often lump various nudibranchs into an assorted category. The result is that numerous species of sea slugs unsuitable for captivity get imported in an effort to acquire just one that eats this species of flatworm.
Manual siphoning is moderately effective but laborious. The truth of the matter is that such flatworms are common and present in many tanks. They will wax and wane and rarely linger in a large sustainable population. If a system is aggressively scrubbed with a protein skimmer, properly fed and not overstocked, the colony of pest flatworms will most likely crash within a couple of months. Support in the meantime through occasional siphoning, increased water flow and a natural predator (if it is suitable for the system and tank mates) is likely to make this frighteningly unpleasant symptom a truly small concern."
