I am no expert on these. I also had them early on and many often do when they start up a tank. When I ID'd mine I really could not find anything definitive on their life cycle, food sources, etc. What I did see fairly consistently is that they do not seem to last long or tend to reoccur once they disipate. I would imagine they starve themselves out after they exhaust whatever their food supply is in the aquarium. I also believe there are different species of these hydroids in the medusa stage so that complicates things a bit further. Some have reported a stinging sensation on their arms after having their arms in tanks with these but it may be that those individuals were more sensative. I think they would only be a problem to other livestock if they were able to suatiain a significant population. They can be easily siphoned out using a siphon line or a turkey baster should you feel the need to remove them or reduce their numbers. I really don't think you will have a problem with these. I cannot recall reading a single post or article that indicated any negative issues with them.
Maybe Amp or Ace can chime in with a bit more info but I belive these are just one of those animals that we just don't have enough information on when it comes to their life cycle in captivity.