I think hzeng33's suggestion is a bit extreme.
Freshwater bath the chaeto if you are moving it to the display - that will kill any flatworms fast. They can't handle the osmotic shock and implode.
That one doesn't look like the typical red planaria that are such a pest, but that doesn't mean you necessarily want it in your display.
I'd freshwater wash the chaeto, put it back in the QT and watch for a while. Might just be an isolated incident. Discard that one flatworm you did see, and when you give the chaeto a freshwater swirly, see if any others fall off. Unfortunately you'll also lose any amphipods and copepods in the algae, but that's reasonable collateral damage.
Jenn