Cold solder is what happens when the solder didn't quite reach the temperature needed to completely melt and do contact, so it looks brittle and would be potentially a faulty connection.

With that said I couldn't spot it in the pic

I don't use a firewall apart from router, but if you set a firewall in all of your devices, the chances of one of them getting infected and spreading it to the others via LAN would be low theoretically.