This also happens to me when using Zoom, Skype and WhatsApp. It is also very common when I start a video call using a web browser (Google Meet, Zoom Web, Skype Web). The camera indicator (green light) goes off only after force-quitting the web browser or desktop application. I don’t think there is anything malicious there, probably a bug somewhere in the code failing to release the camera feed or missing an event notifying the system to close the stream. Even FaceTime fails to stop the camera feed from time to time forcing me to quit the entire app, so it is not just Facebook Messenger.