This MAC address keeps showing up -- A8:BE:27:AC:10:37 -- it’s not mine This MAC address keep showing up on my Xfinity account and my Wi-Fi.

A8:BE:27:AC:10:37
If you use a MAC address lookup tool on the internet it does show up as an Apple device, so there is that.

Some routers allow you to ban devices, if yours does, ban that device and see if anyone in your household complains.

Some routers only allow you to set a whitelist (only allow the devices I specify on), in that case you have to add the MAC addresses of everything you want on your network and everything else is effectively banned.

Again if someone complains you know what that device is.