Id please

[Fishfighter]

 

[Flreefer]

That is Aptasia, pest anemone. This site has some good articles on how to remove it.

 

[Fishfighter]

Why is it a pest?

 

[kdpuffer]

Aiptasia is considered a pest because it grows fast reproduces faster and stings coral and fish. Peppermint shrimp can eat them though it's hit or miss, butterfly fish eat them but they are hard to care for or you can use aiptasia-x, boiling water or fresh lemon juice to kill it. Or remove the rock and burn it with a lighter or small torch.