If that nem is a sebea. Get ready for a HUGE nem. I'm in a 30 gal and moving 2 a 75 very shortly. And it killed 2 rainbow Aussies like urs. Green and red is still alive but barley also a straight rainbow that's returning 2 outerflesh and not just it's mouth.
Mine is a green long tentacle anemone. Your is a monster though.