The phrasing is ambiguous, but the intent is [once each][1]:

> **Ian Tomlin**‏ @zanderkreegan · [13 Nov 2016][2]  
> @JeremyECrawford can a 5th level triton cast each spell they have from their racial once per long rest, or can they cast one of them?
>
> ----
>
> **Jeremy Crawford**‏ @JeremyECrawford · [14 Nov 2016][1]  
> A triton can cast each spell in Control Air and Water. #DnD 


  [1]: https://www.twitter.com/JeremyECrawford/status/798283267468193792
  [2]: https://www.twitter.com/zanderkreegan/status/797988984290996224