I don't think they can fly through the air.
A lot more of us have planaria than would like to admit it, or even know. Not all flatworms are bad and most are just another part of the ecosystem. I've had several different types of FW in each of my systems, none of which ever harmed anything.
It's really hard to make sure you don't come across them. Even if you are dipping your corals from any LFS or online, unless you quarantine and dip every couple days, you might still miss eggs. Or just miss some guys hidden way up in the porous rock that we so value.