What is this?

[Jorshae]

What is this and how do I kill it? Every time I try it multiples and doesn’t die...

 

[GaryE]

Aiptasia. Hard to get rid of

 

[GaryE]

Best bet, if you can get that rock out of the tank. However, if it's been there a while, you will be better off managing the issue.

You may be too late on getting it out completely.

They reproduce very quickly and usually when you're trying to kill them.

 

[Mikedawg]

Search on this blog and you'll find several different strategies. As GaryE says they are hard to get rid of. I see three and bet there are more.

Personally, I use/like Aiptasia X.

Good luck.