If you see an unnamed/nameless device connected to the router this is simply a device that the NETDUMA could not detect what it is.
To find out the device match the IP shown on the device manager to the IP shown in the network settings of your devices. You can then rename the device so you know what it is.
If you have looked at all your devices and cannot match the IP to any of them then it is possible someone has gain unauthorised access to your connection - usually by wireless.
Set an interface password on the router by going to Settings > Miscellaneous and choosing a username & password. Then block the device in the device manager.