What are these white stringy stuff growing from the live rock?

Is this some kind of a coral? Good or bad? I see white strings on different rocks too.

 

hard to tell from pic it looks aipstasia or something similar
kind of hard to tell sorry :\

 

Aiptasia, its a pest. pick up some joes juice or aiptasia x to kill it. it can spread pretty fast...

 

i don't know if its aptaisia since there brown. it kinda does look like a cross between a aptasia and lightless anemone.

 

the tenticles say aiptasia even if you dont see color or a base.

 

They're white and are on the new live rocks that I got recently.

 

I got some on my new live rock that are way bigger than a quarter. I want to leave them to see how big it will get. It wont be a reef tank anyways. Its just going through its cycle.

 

How are they harmful to the tank? I read red legged hermits eat them but mine don't eat 'em.

 

I think the color has to do with the amount of light it gets... ive had them in my tank while fighting off aiptasia, some brown and others clear. brown ones had light, clear ones not so much. Heres a pic of one i had that was on the front of my tank.

 

Its just easiest to get some aiptasia x, it will last for a while if you do it right and do it when you shut off your pumps and go ahead and and nail them all. It can be challenging, but fun. They sting your corals and take up quality chemicals that you put in your tank. you better get him while you can reach him and before it spreads and you cant keep up.