very hard to tell with heavy blue light in pick and video but best guess is acoel. These are the lesser of concern but still must be eliminated as if allowed to grow in numbers can overtake tank and base of coral and block light and reduce production of zooxanthellae which is the food source and buiding blocks of many coral.
If low population, simply siphon up with a net or 3/8" tubing. If quite a bit, consider a wrasse such as tellow coris, 6 line, lunare or melanurus which should eat them. If heavy population, salifert flatworm exit will work but be sure t siphon dead ones daily which can release toxins.
Look daily for 2 weeks for presence of any eggs which are Tiny and scrape off Outside of tank
