IMO chaeto is too cheap and easy to grow to risk getting impossible to get rid of flatworms into a system. I would pull it all immediately, then just toss it and get new stuff.
Alternatively, pull it all out, take a chunk of it, inspect for worms or any obvious eggs, and then do the freshwater dip on that.
There's a bazillion folks here who can give you relatively cleaner chaeto in this group.
Edit: Also since it's already in the tank, the worms are already in that tank. I'd toss the chaeto to avoid it getting into your display (the dreaded 1 week later, "eh, is probably ok"). I'd also plan to do a full reset on the tank (it's a QT, you should be ok to do that).