PraziPro (Praziquantel) kills internal worms, as well as just about all other worms. It is supposedly reef safe, but I personally would be cautious about applying any chemicals to my reef. Also, Because it targets most worms, it will also wipe out many beneficial worms too; this includes bristle worms, feather dusters, coco worms, Christmas tree worms, and maybe cucumbers and other critters as well. I’m not sure about decorative sponges and clams, which can generally be sensitive. But it should be safe on corals, if you want to risk the easy route and just dose the tank. Prazi is one of the few chemicals with this option. I’ve been burned by trusting other supposedly reef-safe chemicals before though.
If possible, I would set up a quarantine, and catch and move all fish to quarantine to begin treatment for several weeks. All could potentially be infected. At this point, you’re probably thinking “oh crap, that’s a lot of effort”. Welcome, you’re one step closer to being a veteran, that QT’s and medicates everything. Stuff like this happens though, even to veterans. We all do our best to take good care of our livestock, but bugs slip through sometimes.
I’d recommend making the decision soon though. Before the worms progress to an advanced stage of infection. You probably have several days, at least, to get started on treatment.