What’s this? Please.

[ChiefZero]

Hi all.
Found this on my LR a long time ago but thought it was alive and little by little it has moved/extended. I’m not really worried about it being dangerous just curious what it is.
It’s a reddish brown and it hard from what I can tell. It moves, though it’s like a tree, you can’t see it moving.
It’s the thing in the middle and kinda looks like a shell from the bottom.

Any help is appreciated.

Ps. I wonder if it’s a worm or a kind of coralline algae.

 

[McDam]

Vermetid snail

 

[ChiefZero]

It doesn’t extend off the rock, just on the rock. Would it be that still?

 

[vetteguy53081]

+1 on vermetid
Vermetid snails form tubes that resemble that of tube worms, and they are actually stationary gastropods whose shells are cemented to hard substrates. They cast out mucus nets from the ends of their open tube shells which act like a sticky trawling net trapping plankton and detritus that passes by.
There are no proven predators for vermetid so reefers should try to manually eradicate them when spotted. While vermetid snails don't always cause immediate harm to reef tanks especially when there is only a couple of them, don't ignore them because they can multiply like mice and become a big problem.
There is some success with control using mithrax crabs, hermit crabs, and coris wrasse or six-line wrasses. A best defense to kill vermetids include covering the ends of their tube openings with superglue gel, reef-safe epoxy, and injecting muriatic acid.

 

[fragit]

What are the little crab legs under that rock? Something you added or hitchhiker?

 

[ChiefZero]

Thanks for the help everyone

 

[ChiefZero]

What are the little crab legs under that rock? Something you added or hitchhiker?

Hmmm. I don’t see what you are talking about but I do have some hermit crabs and an emerald crab.