Acoel flatworms are harmless, so long as you control their numbers. A large number can cause issues especially if they die off suddenly. If you have a wrasse or something I’d just dip the frags then not really sweat it.
Yea I have seen much more aefw come to me, from buying from local hobbiest and reef forum hobbiests, over a small period of time than I have in a large period of time buying wild and maricultured only pieces.