Can someone ID this white worm?

[armandoarturo]

Hi everyone!
There are some little white - translucid worm-like down a new rock I got with a colony of anthelias...
Here are a couple of videos of them... I hope its easy to see them....
Anyone knows what are they ? and If they are harmful pests?

 

[Goaway]

Spionida worm. Usually harmless.