This is not good.
Flatworm Exit can kill them, but I suggest you read the instructions top to bottom before buying, and ask around for other people's experience with the product.
Tank crash is a possibility if these things build up in your system: what happens: you let them get way out of control, until they form huge masses in your tank---you use Flatworm Exit, and immediately they all die, equivalent mass to a freshwater perch gone bellyup in your tank and not removed. What saves you: a good strong skimmer that can yank out the crud as these guys rot; and water changes and probably carbon, after you've used this product. Again: read up on it very thoroughly and ask questions, please!
Yes, I believe a 6 line wrasse will eat them---it will also eat copepods, meaning you can't have a mandy with that fish in the tank, and it may kill smaller fish like gobies.