There are many theories about this.
1) Regina has been to NL and encountered the Mermaid there
2) Mermaids are in both lands (we know for SURE they are in NL, we don’t know they are in FTL yet)
3) Mermaids travel between worlds. Phee can jump in here and tell you the theory about water as a way between worlds.
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"He was a lot of things to me" "The only conclusion was love"