I hope you did some kind of dip before placing them in your tank. I have had them look as you describe and come back. I have also had them die. Place them in a high flow and hope for the best. Also check for any type of critters at night after the lights have been out for at least an hour...
If you don't see any type of snails or nudibranches, I'd just leave them be. Zoos can sometimes act a little weird for a couple of days especially when they have just been added to the tank. Keep us posted if things get worse.
Flatworm exit is a good dip to put your corals through, but once in the system, if you don't know how many you have, they could be potentially dangerous. At least that is what I hear. Once they die the give off toxins.
That sounds like the dreaded SPS flatworms. I'd try and get rid of them naturally. Try a sixline wrasse first and see how that goes. There are also nudibranches out there that only eat flatworms. Once they're gone the nudi's will die. Just a couple of thoughts.