They are Flatworms , but don't Panic!!!!!!!!!
Don't be so quick to put any chemicals in your reef.
I had them, I got a yellow coris wrasse and after I vacuumed as many out of the tank as I could( as per Melev) the wrasse took care of the rest.
I have heard a lot of good things about FWE
but we have all heard the bad thingd too.
KEEP IT NATURAL