OMG!!!..... RUN FOR THE HILLS.....
lol, OK, just kidding, its not the end of the world, and there are great removal products for flatworms, but the trick is to use them before the population gets to big. i read a lot of still where people just said leave them and they will "wax and wane" i never say this, they just waxed, and waxed, and grew larger and more...lol.
i routinely had to suck them out because they are so freaking ugly to look at.
they are not an immediate threat, just an eye sore, adn if they get to large populations they can start to smother corals.
natural predators
target mandarin - mixed success
sixline wrasse - mixed success
blue velvet
nudi - 100% success, but very hard to keep alive
just let me know what other info you are seeking and i will do what i can to help