That is the one issue I've found with flatworms. When I've had them I've never noticed them really harming anything other than the mushrooms. They would basically cover them up and they would starve to death from lack of light, or at least that is my theory. I don't know if the flatworms are really trying to kill them or if they are just eating something the mushrooms secrete.
I was also going to say, if you found one then you do have others, but I see you already found that out. I've used a couple of the different chemicals that get rid of them and I haven't found one brand to be better than another. Just follow the directions. Certain wrasses will also eat them but it is hit or miss if you get one that will eat them. I had a yellow coris wrasse that would eat them. I've read a target mandarin will eat them but mine never would touch them.
