Why is there a spider in my tank?

[new but handy]

I sort of figured there were marine spiders! I was 99.9% sure it was a crab but when all I ever saw of it was 4 hairy legs curling in and out of a hole in a rock. You never know.
There might be a guy coming over today with some coralline scrapings for me he has a mantis problem. I will probably give it to him. Unless you can catch it Rob, I would set up a 10g for it!
For my next seek and capture mission, the 10" bristle worm that I finally saw in full last night.