Jonathan,
Flatworms....
The bane of all reefers. All I can say is. Make sure everything you put in your tank is coming from a system that HAS NONE. Never add anything to your tank that has flatworms. Even if they tell you to fresh water dip the coral to kill them off. It isn't a guarantee that they will die.
Read this links for some info and a pic of these critters.
Flatworms