Whats the deal with flatworms?

There are a few wrasses that will control FW populations, but not are the lined types. 6 lines will not generally eat FW's, but every once in a while you will see people report that their 6lines are eating them.

You will need a halichoeres wrasse (a genus of wrasses) to control FW's. I would actually go out and say that they destroy FW's..

christmas wrasses, melanurus, wrasses, yellow coris wrasse (not any real coris wrasse though), neon wrasses, neon green wrasses, raidant wrasses are a good few of the more popular ones.

The addition of a halichoeres wrasse will not only keep FW populations in check, they will also eat pests like pyramellid snails that prey on tridacna clams.

I would recommend that pretty much every reef tank have a halichoeres wrasse in them to keep these pests in check.