Can one app access an app that has password protected?
No, The other app wont be able to access this app but other app can access this app data if its not stored encrypted on device storage. Other app only need storage permissions to do that. If other app can access device storage then that app can access any unencrypted data stored on device. But it wont be able to access OS files which are stored in read-only partitions/folder without having root access