Red planaria. The worst part about them is that they will reproduce to plague proportions smothering things.
AFAIK they don't eat coral or anything ecspecially bad
I control my population of them with a spotted mandarin and a melanurus wrasse. I'm not sure which fish eats them because I got the two fish at around around same time and almost immediately stopped seeing the flatworms in the display.
You can siphon them out and then use a chemical to kill them. Just be careful and read up about it because when they die they can supposedly release a small amount of toxin, which isnt too big of a deal until a ton of them die all at once. Make sure to read up about it if you go that route.
How is it going controlling yours? I sucked mine all out and have not seen any in the display and just ordered a M Wrasse to keep in the DT not sure if I should do anything else for now or not....