Hey Anthony,
I feel for you... I have some HUGE bristle worms, and having read that some may take swipes at seahorses, I've been tempted to just take them out in a massive Panacur nuke, (unfortunately, you will lose your other worms as well, but hey, they'll come back
). It won't harm the seahorse, (Panacur), but will take out your hydroids and worms. Gaging by the rapid success that Dyereefer has had, ( I haven't dosed my tank yet), I'd say, you could be a day away from feeling happy again.... although, I'm not sure if the seahorse will last that long
. The only part that concerns me is, even if you kill the worm, while in the seahorse, there is no guarantee that the seahorse will be fine... but, do nothing, and there is a good chance he won't be. Not a good situation to be in, that's for sure. Depending on where you go, Panacur can be really cheap, (the vet clinic I went to gave me about 10 mg for free, but, I did need a prescription for it, though).
Let me know if you go that route and how it turns out, (although, I'm quite surprised he didn't just blow bristle worm chunks out the back of his head, like they do when they take in more than just their food). Good luck.
Robert