Research on Pycnogonids ("sea spiders") is pretty sparse, at least when it comes to specific species that infest zoa's. Some Pycnogonids do have parasitic larvae that embed in their host and this may be the case with zoa's. Others have claimed to see eggs in the polyps. You will need to keep them in QT for quite some time to make sure they are free of issues.
A thread on Reef2Reef talking about seeing spiders in the polyp here:
https://www.reef2reef.com/threads/t...ers-with-0-1-milbemycin-oxime-how-to.1106144/
The spiders can live for months, and some claim they can live without zoa's as their primary food source for a while. That is why they are so feared/hated.
If I was in your place, starting over or not would depend on whether I wanted zoa's in the tank and how old the tank was. First coral you put in it? And you want zoa's? Maybe I would start fresh. Conversely if you can go without zoa's, I would keep going.
Good luck.