Yes, flatworms.
If they are few in number, manual removal can help keep them in check. You can also do a temperature adjusted dip of your corals that can be removed in RO/DI water. (10-15 seconds, swish the coral around a couple times, the FW's fall right off)
Flatworm Exit is another choice - one I found to be ineffective, even at 2x strength.
What finally did it for me was a pair of Blue Star Leopard Wrasses. They have completely rid my system of flatworms. As an added bonus, they are very peaceful, stunning to look at, and spend their days cruising around looking for little things to eat.