What are these white stringy things growing under rocks?

[GGhana]

 

[Chriscorals]

A type of sponge if I’m not mistaken

 

[death-by-reef]

def a sponge, r u using an rodi

 

[damsels are not mean]

sponges or some other filter feeder, harmless. Old rocks will be covered in this.

 

[Twocorns]

Vermitid snails?

 

[Jeeperz]

Sponges

 

[Lbrdsoxfan]

Pineapple sponges

 

[Jwise]

That's sea sponge and its actually good that you have this because its a sign of a mature reef tank which means you did a good job at keeping up with your tank and its parameters.

 

[vetteguy53081]

Poriferan sponge which favor dark, low flow areas. It will continue to branch out although slowly. If you want it gone, expose to air or inject with vinegar via syringe