On Android Oreo you will get a notification message saying AccuBattery is running in the background / using a lot of battery. This is something they will show whenever an app runs persistently in the background, regardless of how much power it consumes. AccuBattery needs to run in the background in order to track your battery consumption, we've minimized the power we use when doing this and you can check in the system settings under battery/power consumption to confirm that we indeed use very little power.
In Android 8.0 it unfortunately looks like it's not possible to disable this message in the settings and you'll need to use a third party app: https://9to5google.com/2017/09/06/how-to-remove-android-oreo-background-app-notification/
In Android 8.1 you can hide these kinds of messages for each app via the settings menu: https://android.gadgethacks.com/how-to/android-8-1-oreo-lets-you-hide-running-background-drawing-over-other-apps-notifications-0180752/