That's a flatworm. Nasty buggers. There's flatworm exit, but they're toxic so you have to be ready with carbon and a large water change so your corals aren't affected. I'd treat three weeks in a row since some always manage to survive. Also, you may need to double or triple the suggested dosage depending on the worm. Best way to test is to get a gallon of tank water, put in some flatworms and add a drop, wait, add another drop until they're all dead. Then you know your drops per gallon. At least there's a treatment. There are tricks to siphon out as many as possible before you start too, DIY tools and attracting them to a flashlight during a lights-out period.