Generally speaking for the most part at that size it will be in the Polyclad family to help start narrowing your search. If I had to guess by this photo and if you say you think it was a flatworm meaning saw it moving etc looks like possibly a Leopard Polyclad.
Thank you, after some google I definitely think it’s some kind of leopard polyclad. That would also explain my intermittent snail disappearance which I had figured was normal.