flatworm identification

Hi reefers, wondering if anyone can help ID flatworm. Found them when dipping my new acro in quarantine system. I’ve been dipping them weekly for almost 2 months, and there are still some fell off after the dip. I think they’re acoel as their have swallowtail, but would like to confirm it.

I believe I have some flatworms on a frag and have done a freshwater dip to no avail. They didn’t fall off so I’m not sure exactly what I should do I did the dip about 3-4 days ago.