I am still a newb myself but I did just recently read an article on flatworms in Reef Invertrebrates by Abthony Calfo & Robert Fenner. One kind of flatworm is called Acoel. There are two very good photos of it and they look identical to what you have as well as matching your description.
They are common and are generally a non nuisance. It is possible however that under the wrong conditions they can multiply rapidly to reach "plague proportions."
If this happens the best course of action according to the article is to "focus on water changes, manual siphoning, improved water flow and aggressive protein skimming."