Actually, flatworms, as a general rule, aren't a good thing to have in your tank... although this particular kind isn't as bad as some of the others.
Pods are tiny little shrimp-like crustaceans that live in your sand and live rock. They consume microalgae and detritus and are a beneficial part of your tank's infauna clean up crew. There are three different types... isopods, copepods and amphipods.
If you shine a flashlight into your tank after the lights go off, you'll likely see what looks like little bugs running/swimming around on your rocks and sand... those are pods.
HTH