now you mention it , it does look a bit like javamoss.
its clear though and more of a jelly like thing and if you think of javamoss it grows its "leaves" close to the main branches and on both sides, whereas this stuff seems to grow its "leaves" on only one side and a bit further apart.
maybe it is linked though , would be interesting if anyone has any more info
It could be a species of dinobryon. Under a microscope it looks like small colonies of flagellated protists. It's a form of golden algae. I had a ton of it growing on my tank, but nothing that macroscopic.
The only tanks I've had this growing in filters all contain pond snails. I've always assumed it was left by them. Possibly their egg masses being spread out by the water movement?