Well I didn't see any flatworms in that pic. If this is one of the typical Flatworms, they will not harm corals IME. They are actually beneficial to the system (eat Algae etc.) it's just that they tend to do very well in some tanks and large colonys form making them pretty much eyesores only.
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One Reef to rule them all, One Reef to find them, One Reef to bring them all and in the Depths bind them.
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