Generally speaking, flatworms are not things you want in your tank (ghost flatworms are pretty well the only harmless ones I know, and even then, they're usually not wanted) - I'd make sure to get rid of any you can see, and I'd take a close look around for any eggs on the coral. If you find any eggs, get rid of those too.
Here the States, Xenia is a common enough coral that I might just suggest dumping the frag and getting a new, clean one, but I don't know how plausible that would be in Serbia.
Either way, many wrasses will eat flatworms, but if the flatworms get out of hand, people often have to resort to chemical methods to get rid of them - since the flatworms release toxins when they die, this can be a bit dangerous for other living things in the tank.
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