Port forwarding - keeps adding random MAC address.

[#1]

I'm setting up my CCTV so that it can be accessed externally. I've set up the correct ports to forward and everything worked fine. Then I logged out of the router and next time when I logged in I found that my port had an additional MAC address underneath and now my port forwarding doesn't work.
Anyone know where this random MAC address is coming from? Plusnet live chat can't help as they're not trained on this.

[#2]

Have you perhaps setup some static IP addresses with the DHCP range of the router and you're getting clashes?
The first MAC listed is from a device made by Ingenico - Do you have a payment terminal on your network?

[#3]

The DHCP range stops well short of where my static IPs start so I wouldn't think so. I'll reduce my DHCP range a bit though, just to be on the safe side.