OMG those are simply beautiful!!! I'll follow along hoping to get a positive ID so I know what to look for on my next purchase!!!
As for the Flatworms, they're common Planaria. They aren't harmful until they reach such a high population that they cover your corals and block out light. The other problem with them is that they become toxic when they die. Sooo, if you treat with chemicals, which does work and is safe, make sure you get as many out beforehand and afterwards, syphon up all the dead bodies.
I'd suggest starting with a filter sock on your drain line, if you have one. Otherwise, use a filter sock on the end of a siphon hose, siphon out a lot of them and you can add the water back in. It's also been suggested that low flow can cause an outbreak of them.