Aiptasia, and a big one at that. There are many different methods of removal, and you'll find everyone has had varying degrees of success with each. If you've caught it early enough, you may be able to kill that nem with aiptasia-x or lemon juice. If it has had time to reproduce, then there is a good chance they are all over the tank and youll need to introduce Berghia Nudibranchs , filefish or true peppermint shrimp. Its better to take action now against the nems you can see instead of waiting, as they can spread into plague like proportions alarmingly quick.