Try looking up Convolutriloba (=Convoluta). You aren't really in any trouble. They are just unattractive. They aren't going to harm anything in your tank. Some people have them go away on their own eventually, just like any algal bloom. Others use something like flatworm exit to kill them. They do contain endotoxins so if you use flatworm exit read up on it heavily beforehand and be ready to use activated carbon to absorb the toxins from dead flatworms out of your tank.
HTH,
Kevin