Birdsnests are fast growers, but also very sensitive....especially to flow. More of a semi-beginner coral IMO. What you're seeing is pretty typical for birdsnests....though it's not necessary.
What happens to most branching corals in marginal flow situations is that the grow out, that growth has an impact on flow – literally blocking and reducing it – and then they (or other corals around them) start to suffer from too little flow.
This impacts lots of things, but maybe foremost of them is nutrient and oxygen uptake – feeding and breathing. So RTN becomes a likelihood.
If you improve flow (make it stronger) it should stop losing tissue.
Unfortunatley it won't regrow over the bald patches and that area will now be prone to algae growth. If it grows coralline algae, no big deal. But if it's hair algae (or other) then the coral is basically toast since there's really no way to get in there to remove the algae. In that case frag it before the algae gets too established – do not try to wait it out.
