What is this? Is it good or bad?

[Patty's tank]

I need help to ID what this is! I bought 10 nassarius snail 2 days ago from my LFS and today I noticed this growing on one of the shells. Does anyone know what it is and what I should do? Thanks in advance

 

[kashman100]

Aptasia

 

[Patty's tank]

Awesome! Thanks for your response :)

 

[Salty1962]

Does it retract when touched?

 

[Patty's tank]

It did when I used AptasiaX on it!!! I was able to kill it with no problem and it did not multiply... thanks for the response