I have a question for other ORA green polyp birdsnest owners (seriatopora guttatus). Does yours die off in the middle of the colony? My colony right now is about 5"x5" and the inside of it has died. I think this is due to lack of light getting to the middle. At night the polyps seem to close up a bit and you can see into the middle but during the day they open up to the point where they block light from getting to the middle of the colony. It is almost as though they are trying to "self-destruct" or something. My colony is in good flow and I can tell that the flow does still make it to the inside of the colony so I'm pretty sure it isn't due to flow.
I know it was mentioned earlier in this thread that some people's birdsnests seemed to grow like a weed and then slow down. My take on this is that if the middle of the colony dies, the entire colony is no longer linked together. It becomes a bunch of frags (outside branches) connected to each other through a dead skeleton. So each frag has to fend for itself and doesn't have the colony's total energy reserves to use for growth. This could explain why the growth would slow down. However, mine is still growing just as fast as it did when it was smaller and the middle was still healthy.
What puzzles me the most is that I have a pocillopora damicornis that is the same size. The branches are a little farther apart but when the polyps are fully extended during the day they block just as much light from the inside of the colony. Yet, this colony still has good tissue, color, and polyp extension towards the middle.