Some times green indicator light blinks for just a moment even the camera is not used by me, specially once I disconnect or turn off wifi for control centre on my iPhone 12 pro max!

"A green indicator means either the camera or the camera and the microphone are being used by an app on your iPhone."

• Status icons and symbols on your iPhone - Apple Support

Oh I get this too it’s nothing to worry about I don’t think, this usually happens when a app you use opens into a camera state straight away, always happens with Snapchat for me just briefly.