Accha is a Hindi word meaning "OK," "good," or "I see," widely used in South Asian conversation, including by English-speaking South Asians as a discourse marker. Not concerning, just bilingual speech. 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.