Overlay DHCP network appearing in simple mode

My Firewalla Blue is working in simple mode on my ubiquiti Unifi network. Everything is functioning perfectly, but I'm seeing the Firewalla advertising the overlay 192.168.218.x network which I didn't expect unless it was in DHCP mode.

Here's the abbreviated network config:

eth0 Link encap:Ethernet HWaddr 02:01:90:47:4d:49
inet addr:192.168.20.105 Bcast:192.168.20.255 Mask:255.255.255.0

eth0:0 Link encap:Ethernet HWaddr 02:01:90:47:4d:49
inet addr:192.168.218.1 Bcast:192.168.218.255 Mask:255.255.255.0

Again, it's working fine, but filling my Unifi USG logs with security alerts due to the unexpected network. 

Is this a known bug, or by design ?  Can I simply down the eth0:0 virtual interface ?

Thanks,

Damon