Good question. I know I had a nice wild piece years back when the first one was becoming popular that was just like it. Looked very similar to this but in this picture the tips are much longer. Mine original one also tabled really thick. It could be flow and tank conditions making it grow different and or a different species. I would like to hear the answer too.
When I said slow grower over a year ago, I wasn't kidding. (The picture in the OP is mine that I posted Feb 2018) The thin branches are getting longer and it is still slowly growing in diameter. I have taken a few frags, no more than half a dozen.
Still no idea the species though.
Here is a picture from today along with it to compare.
And a frag that is showing the same growth tendency
You can see in sections of this colony the long axial shoots coming up. Not sure what it takes to create that growth, but I've seen it a few times in the hobby and a few times while collecting as well.