I do not have any personal experience with NPS corals. HOWEVER, my initial impression is that it is some type of sun coral or something similar in the Dendrophylliidae family. The color definitely reminds me of duncans, but the shape just isn't quite there and is much more like a sun coral in my opinion.
You could try offer it food and see how it reacts.
If it is indeed NPS, it may require regular hand feeding and you will want to place it in the shade as I understand that NPS typically do not have any defense against algae, so you want them in the shade where algae won't grow.