Any idea what's on my live rock?

[kaixuan1995]

there's quite a lot on my rock, it's not moving.
Anybody can tell me what's that? Are they harmful for marine tank?

 

[Jason mack]

Tiny koker Worms .. filter feeders

 

[kaixuan1995]

Tiny koker Worms .. filter feeders

Is it good for the tank?

 

[reef lover]

Do u see them cast out nets of mucus?? Could b vermetid snail which aren't so great they bother corals and spread rapidly.

 

[Mikedawg]

Koker worms: vermetid snails:
Body casings look very similar, difference is the head with vermetids having a mucus net they use to capture food; I'm thinking vermetids.

 

[ajm83]

Body casings look very similar, difference is the head with vermetids having a mucus net they use to capture food; I'm thinking vermetids.

Same here. On the right of the OPs pic you can see one with the mouth parts showing.

Some people don't care about them, but I hate them. Ugly and they make it difficult to pick up rocks without stabbing yourself.

There have been studies which show that they slow growth and damage corals in nature, not sure whether that applies in a tank though. .