yeah I did flatworm exit about 4 years ago on my first tank
the only way I got flatworms is because I got an anemone on a rock that was infested with them, so I separated the anemone from the rock and left the rock behind...the rock has never touched the tank
so I must have had them from something prior that I didnt notice, or else some hitch hiked on the anemone
whether or not they bother anything, per say, they bother me by covering the glass with a red tint
I have also read about the leopard wrasse potentially eating these things