Unfortunately, once you have a population of these pests, it's difficult for any Wrasse to consume them as fast as they multiply. A Wrasse may slow down the growth rate, but the population will still rise.
I'd suggest siphoning them out every couple of days, until you get the population down some. Keep in mind that there are MANY more than you can see.
Also, if you do use Flatworm Exit, which works great, DON'T underestimate the toxicity of dead Flatworms!!! The toxins they release at death can wipe out your tank if you aren't careful. But don't let that keep you from treating. There are steps you can take to minimize the risk and if the proper steps are taken, treatment is quite safe.
Siphon out as many as you can BEFORE treatment. Follow the directions closely, except for dosing. IMO, the does isn't strong enough. When I treated, and from other experiences I've read about, X2 dose works great. The Flatworm exit is reef safe, so using a larger dose is safe. It's the toxins of dead flatworms that aren't reef safe. Have plenty of fresh carbon ready to go and have a way to use it, other than a filter sock in your sump. You want water to be forced through the carbon. A Canister Filter works great for this.
If you have a sump, put a filter sock on your drain line to catch as many dead flatworms as you can, after treatment. Siphon dead ones out of the display as you see them. Use a turkey baster to blow them off of live rock and glass.
By the way, what kind of wrasse had you purchased and what kind of anemone do you have?