Yes they are definately flatworms. They eat dead coral tissue, which sound like a good thing, except that their big flat bodies results in that part of the coral to not recieve light actually causing it to die. In big enough numbers they can pretty much block out all the light from your corals... Not to mention, they stress out corals and release toxic chemicals when they die or are killed. You can gamble with critters that eat them but I have personally tried a six line wrasse-he didn't touch them, a spotted mandarin-if he ate them, I didn't see it. I used a turkey baster to remove mine, luckily I didn't have many and I caught it early.
I use Flatworm exit as a dip not and that stuff is incredible!!
Oh, lemme show you... These are my progress shots of a coral given to me by a friend who thought he might not have had enough light for it. It was dying off branch by branch in his tank, so I took it home...