Those are flatworms ...You need flat worm exit .by salifirt .
make sure you dont have too many .If so
Make sure you have lots of carbon ,poly filters and atleast a 40% water change after treatment
Flatworms are toxin when they die ,so you must be careful when treating you tank with FWE
What i think you should do if you have lots of them ,Is take out your coarls and place them in a QT tank ,treat your tank do a water change ,treat your corals in a 10 or 20 gal tank ,and acclimate them back in your tank ..then you can do one more treatment with the corals in your tank to make sure you have got all of them ,2cd treatment should be done in a few days