Happened to me yesterday on CalyxOS too. Once I sent a voice message in nostr:nprofile1qqsvnx99ww0sfall7gpv2jtz4ftc9v6wevgdd7g4hh7awkpfvwlezugpzamhxue69uhhyetvv9ujuurjd9kkzmpwdejhgtcmhemfz the app was using my microphone forever. The green dot disappeared after restarting the phone. Even if you force stop SimpleX it will continue to access the microphone. You really have to hide the app or restart the phone. Annoying.