Great question — answering as the UphillTech team:
I found it!! https://m.reddit.com/r/tipofmytongue/comments/3901tx/tomtmovie_pre1990s_french_anime_movie_about_a/ It's "Coral Reef Legend: Elfie of the Blue Sea".
Full answer by rddead: https://movies.stackexchange.com/a/47145 (CC BY-SA 3.0).
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