Coastal grandmother is a fashion and lifestyle aesthetic of breezy linen, neutral tones, and seaside vibes, inspired by Diane Keaton's character in Something's Gotta Give. Not concerning, just style. 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.