The app displays a persistent icon in the notification area. This is done to increase the process priority, and thus important for normal operation of the app (see Best practices to avoid issues with call recording for more details). All in all, it’s highly recommended to keep this icon enabled, but if you don't like it, select “Hide icon, but keep notifications.” The notification drawer will still be there, but the icon will disappear. Even if you turn notifications off, the notification drawer will be accessible while the app is recording.

Android has more options to control notifications. You can read about it here: https://support.google.com/nexus/answer/6111294?hl=en