Geocentrophora planaria flatworm. Assure there are no others o presence of eggs. Planaria are a variety of flatworm that are considered a pest by the majority of aquarists as they can quickly multiply in number and smother coral. Defense is to remove and/or siphon. Dont crush or pinch them to death as these worms if cut into 5 pieces can regrow into 5 different planaria worms !!
These guys feed mainly on waste in the tank and regular maintenance will lower their population and life cycle.
Some wrasse will eat them such as malanurus, 6 lined and lunare wrasse.