It's funny. Saltwater keepers know the benefit of a complete ecosystem in their tanks, but freshwater people freak the moment something that isn't a fish appears. What you have are natural components of a healthy aquarium. You should have dozens of living animals that are either invisible or barely visible. Some are worms, some are crustaceans, but they are almost always helpful. They generally eat algae, fish waste, and smaller critters. If you see them on the fish, then you have a parasite.