said by graysonf:fe80::201:5cff:fe22:c9c1 is my IPv6 gateway.
There are occasionally other entries in the system log not related to IPv6, but the one cited appears at least 160 times every ten minutes in the form of "last message repeated 173 times."
In that case, I can't imagine why m0n0wall would be logging that message so often. I only see the IPv6 RA messages in my syslog during a reboot or DHCP renewal time frame (and I have debug logging enabled since the DIR655 is a new router that I am currently still evaluating).
Such an intensive logging level may just be normal for IPv6 in m0n0wall if you are using DHCPv6-PD. My D-Link router may also be seeing additional RAs from Comcast's gateway, but just simply ignores them (and doesn't bother to log them) if they are unexpected (and unneeded). If my curiosity were roused enough, I could probably run my DIR655's WAN through a VLAN segment on my Netgear GS108e switch with a monitor port configured so that I could capture packets to/from that interface (but it would be a PITA to physically setup). Does m0n0wall have a built-in packet capture capability? I know that BSD can do it, but I am not sure what resources are available on m0n0wall's implementation.
Hopefully you will get some information from the vendor's support site. You might also try posting your question in this site's »
Unix and Linux forum; maybe some of the regulars there have seen something similar.