It is a flatworm. You should use some Flatworm Exit. Follow the directions on the package (it's okay to use more than the recommended dose if you don't see them dying right away), and be ready with a water change and some carbon.
They are toxic in high enough numbers. Some will eat corals. And they are ugly. They will continue to multiply until they cover your rock, sand and glass. There are fish that eat them -- wrasses, for example. Since you have FOWLR tank, you can add one of the non-reef safe wrasses that are known to eat them.
+1 biff....I had some flatworms (might still have some), but they're kept in check by my 6-line wrasse, I'm assuming. I don't see them on my glass at all. Usually when you see them, they're already rampant. I'm FOWLR, too.
It is a flatworm. You should use some Flatworm Exit. Follow the directions on the package (it's okay to use more than the recommended dose if you don't see them dying right away), and be ready with a water change and some carbon.