The phrasing is ambiguous, but the intent is once each:
Ian Tomlin @zanderkreegan · 13 Nov 2016
@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
A triton can cast each spell in Control Air and Water. #DnD
