What is this? A pest?

[bennycbenny]

Hi everyone,

I have these small circular white things on the back of my tank and probably elsewhere too. Do you know what these are? The picture attached is one I found online (talking about Vermetid snails) but I think they are referring to the larger thing on top of the frag plug. What are all those small white circle things? And are they harmless?

Thanks!

 

[Sharkbait19]

Spirobid worms, harmless filter feeders!

 

[bennycbenny]

Spirobid worms, harmless filter feeders!

Nailed it. Thanks!