I disagree with Mary - this may not be harmless. If the device is showing in your network list, your router has assigned it an IP address and it has access to your network and everything on it (depending on your internal network security settings). You
really should investigate this further.
I have had the same issue recently - apparently a recent update to Android caused a cheap D2 tablet that I have for my kid to show up as a Phone on my network (whereas my Samsung and Asus tablets do not, nor do my iOS devices). Took me a while to chase
it down based on MAC address, but finally found it.
Now, if you don't have an allowed Android device, it is likely someone is busting into your network (watch for cars out front of your house, or a person on a phone hanging out nearby), and this is obviously a problem.
Check your router for the option to Blacklist the offending MAC address to prevent it from getting an IP address (although most routers use a white list approach where you have to allow each device in your home). Blacklist is so much easier and worth
a router upgrade in the case IMO. Some parental controls include a MAC filter, where you can disallow access based on a MAC address.
Changing passwords and SSIDs may work temporarily but it is pretty "easy" to hack WPA security if you know what you are doing or have a cheap tool from the internet.