How can i get or detect FCM notification while app running?

You can create a service that will receive all FCM messages and then you can react on the received messages accordingly to show a notification.

https://firebase.google.com/docs/cloud-messaging/android/receive

public class MyFirebaseMessagingService extends FirebaseMessagingService {
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        Map<String, String> message = remoteMessage.getData();
        // do something once message is received
        }
}

In your manifest:

<service
    android:name=".MyFirebaseMessagingService">
    <intent-filter>
        <action android:name="com.google.firebase.MESSAGING_EVENT"/>
    </intent-filter>
</service>