Mermaidcore is a 2023 fashion aesthetic of shimmery fabrics, sea-green tones, shell jewellery, inspired by The Little Mermaid live-action film. Not concerning, just fashion. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.