I've seen them in my Refugium from time to time. They are Not Flatworms for sure. I've never been able to find out what they are. I did some research a while back for 2 hours with nothing found. All I can say that they are jellyfish like. They haven't done anything bad in my tank so no worries from me... oh well

http://www.wetwebmedia.com/jellyidfaqs.htm