Looks like a zoa eating nudi. If you have zoas check them for eggs and/or more nudis. The eggs will be on the stalk of the zoa and are white. The nudis tend to take on the color of the zoas they’re eating, so they may be hard to spot.
When I had them all over some wild colonies I bought a few year ago I did a fresh water dip and shake of my zoas every 4-5 days for about three weeks before they were all gone. There may be other methods of removal that hopefully others chime in with.