Those are super bright, which is kind of surprising. That said, they're either colonial hydroids or Nausithoid jellyfish polyps (probably from the Nausithoe genus - I'm currently leaning toward jellyfish personally); sometimes they're invasive to the point of tank breakdowns, other times they don't seem to spread much at all - in both cases, though, they reportedly do have a nasty sting for some people:
I found some of these in my tank- and I'm really not convinced that they're colonial hydroids. Every single ID thread I've come across has them labeled as colonial hydroids, but based on a few observations I've made, I'm not convinced. Here are some pics of the exact type I'm talking about...