Pycnogonida?

[commod0re]

This little guy hitchhiked his way into my wife’s new macroalgae tank. Is it a sea spider? We pulled it out just in case

 

[Gumbies R Us]

@ISpeakForTheSeas

 

[ISpeakForTheSeas]

@ISpeakForTheSeas

Definitely a sea spider of some kind, yes:

Sea Spider ID can be pretty tough and I don't get to do it very often (probably a good thing), but pics of the body parts around the proboscis would be needed for specifics. Also, OP, if you find any carrying eggs/babies, info on that is sometimes useful for ID as well (it can sometimes help tell if the specimen is male or female).

I can't see the proboscis at all in the pic, but each sea spider has a proboscis; from there, they can also have any or all of the following, sometimes dependent on sex:

-Chelifores
-Palps
-Ovigers

(For examples/explanations of what those are and where to find them):
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7826184/figure/msaa228-F6/Depending on which of the above are present on a pycnogonid (a sea spider), we can narrow it down to a taxonomic family. I don't remember offhand how to narrow it down to genus/species from there, but I could figure it out again if needed.

 

[Michael Hughes]

Yes, definitely a sea spider. Such an enigmatic group; some are known to be incredibly destructive in a reef tank, destroying LPS and SPS without reservation. Others in the wild are detritivores, and others are generalist predators.

With the little that's known about these, and difficulties with ID beyond Pycnogonida, I would probably play safe and remove any sea spider.