Tiny white worm with black stripes

[DrNickk]

I noticed these 3 small worm like creatures in my tank, coming out of a rock. They seem kind of stiff and move just a little bit in the current. They've stayed the same size for the past 2 months. I saw another person post something similar but didn't get answers. Is this most likely harmless?

YouTube Vid

 

[IceNein]

Micro brittle starfish? Some kind of echinoderm, I think. I have some that lurk inside little crevices in my live rock and stick their arms into the water stream trying to catch their prey.

But as always: @ISpeakForTheSeas

 

[DrNickk]

I noticed that if I touch them with something, the arm will instantly go limp and lay against the rock. Then it slowly stands back up after 10-20 seconds. YouTube vid 2

 

[DrNickk]

Ok it’s definitely some type of banded starfish!! I blew water at it and it disappeared for a day, then reemerged about 6 inches away, still with 3 tentacles extended. Is this good or bad in the tank?