yeh, flatworm, but not the red kind that seem to multiply quickly. keep nutrients low and these shouldn't be a problem...may live on mushrooms, but probably not hurt them. see these a lot in fresh tanks, then they seem to go away. siphon out what ya can.
Looks like planaria flatworms. They are relatively harmless in small numbers, however they can quicky multiply until they are out of control (smothering tank inhabitants). There are a number or wrasses, damsels and mandarins that can act as natural predators. They can also be manually removed by siphoning them out of the aquarium. There are also a number of products that will eradicate these as a last resort.