Yano, I think you may have a form of medusa worm, closely related, if not actually the same thing as a spaghetti worm. it is a creature beneficial to your aquarium. How big is your tank? If you are worried about aptasia being a pest, I would ask Instar if he can spare a few berghia. instant solution, your pest becomes a food source.

check out hte pictures at the bottom of this page, they are pics of medusa worms that have come out of their hiding places.

http://www.wetwebmedia.com/wormidfaqs4.htm