Weird Worms

[Jsscahd]

Sooo I haven't been able to conclusively figure out what this is, answers have varied widely. What are these things? Hasn't harmed anything yet and I thought it was just a type of algae, but last night I saw a worm poking out of the rock near it. It seemed to be searching for something, poking it's head around. I have the little sand pillar worm things under the same rock, but I don't think they're related. Just want to make sure nothing harmful is in my tank, I haven't seen this worm inhabitant until last night and I don't want him wreaking havoc on my currently thriving reef tank. I have quite a few corals growing on this particular rock

 

[sixty_reefer]

They are hydroids, semi invasive this species

 

[Rick's Reviews]

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[HankstankXXL750 Verified Sponsor]

Here is a pretty good website with hitchhike identification.
https://saltwateraquarist.com/reef-tank-hitchhiker-id/