Not an Atlantean
Since Clockwork informed him that his wail had evolved, Danny decided that he should try it. But he couldn't do it on land since he would destroy the whole place so he flew to a desolate place in the sea. He was sure that he could test it without destroying anything there.
When he finally decided to use it he noticed that his wail had sung to some sort of cry of suffering. Which wasn't much different from before but it made him sound like a wounded animal. Danny frowned wondering if that meant the old ghost with "Banshee".
To the citizens of Atlantis, Danny sounded like a wounded merman. But while some features fit right in as soon as they peeked out of the water (the fangs, the green eyes, and other features), they noticed that he had no tail...maybe he was only a descendant?
Arthur, excited to teach the human about his true ancestry, came out to congratulate him. The halfa was very confused when they led him to Atlantis. Noting that the boy could perfectly survive underwater, the citizens of Atlantis "confirmed" their suspicions. The boy was a descendant of their people, but how could they ask him without bringing back bad memories?
On his part, Danny was very confused, but everyone was extremely nice (he didn't notice that his ghostly features could easily be mistaken for another creature). Honestly, the halfa was more interested in asking how the stars looked like from the open sea.
After a while Danny noticed that something weird was going on (everyone was too shy??) and Arthur kept talking about letting him stay in the palace and learning directly from the King. Well, the halfa still didn't know where he was or what was exactly happening but seeing a King would surely help him when he was crowned so what could go wrong?