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[waldoz]

I'm thinking he is some sort of filter feeder.

[SarahJ]

Could anyone please tell me if this is a bristle worm or fireworm?

thank you

[Dewi]

So I’m extreamily new to this hobby and you just answered tones of unknowns thank you so much fantastic post 

[Oldtimer65]

Hi. I have a 16 gallon Biocube that has been set up for about 4 weeks. Used well established cured live rock and live sand. Never had an appreciable cycle. I have these odd hitchhikers that seem to be having a population explosion. They hang on to the hair algae and send out 2 tentacles into the current that look like feathers. I’m hopeful someone can point me in the right direction to identify so I can decide if they need to go. I’m including pictures. They do move down deeper into the algae when they aren’t feeding. They reel out the tentacles into current and then pull them back in. 

Edited November 1, 2023 by Oldtimer65
Added more information

[Tired]

Interesting! Do they react to touch at all?

 

Likely not a problem, I'd think. Filter-feeders and detritus eaters very rarely are, unless they frequently touch and therefore irritate corals as they feed.