What is this growing out of my live rock?

[New hobbyist]

What is this growing out of my live rock. They are almost like tentacles with white spots on the end. The tentacle things sway in the water but the whole thing just stays put. It doesn't travel around on the rock. Does anybody have any idea what this could be? And is it good or bad?

 

[bribo12]

Corynactis nem aka a ball anemone. Definitely not harmful and really cool.