Help. What is this ?

[underwaterdan]

Saw this pop up for the first time today. What is it?

 

[cloak]

Aiptasia. (not good)

http://reefkeeping.com/joomla/index.php/current-issue/article/33-reefkeeping-101-

 

[DonTavo27]

Yup not good, but don't panic. Look into buying some aiptasia x to get rid of it