If something looks as human as a mermaid, nobody would let the scientists treat it as an animal without a huge amount of evidence that it is, and even then newspapers would print pictures of mermaids posed to look as if they're crying for being kept in a tank.
And remember that the stone changes reality. If Nathan is a mermaid, the most likely changes in reality are either that he was there all along (in which case nobody would be surprised that he suddenly appeared) or that something turned him into one. This could mean that he has shapeshifting ability (in which case he can change back) or it could mean there's something in the room that makes people into mermaids.
Besides, this started with Jon gaining the ability to become a mermaid and doing so under his own power, not the stone zapping him into one directly. I'd expect that to mean he can change back.
