It sounds like the webcam has its own built-in web server. It also sounds like it is using an internal private IP address and that currently you have all port 80 traffic being 'forwarded' by your router to your Mac server. Typically a router can only forward a specific port number to a single destination, so either you need to have it forward all port 80 traffic to the webcam and then not be able to access your Mac server, or you need to set the webcam to run on a different port e.g. 8080 and have that forwarded to the webcam and access it via that port. Most devices acting as web servers will have the ability to change what port they use because of this exact sort of situation.
Something that might help make things easier for users would be to define multiple host names on your Mac server, one would be the main one for the Mac server itself, and another would be for the webcam. It is possible to define additional web server 'sites' in the web server on the Mac so you could define one for the web cam which listens on port 80 but then immediately redirects users to the different address and port of the webcam. I do this myself on one of my Mac servers although not for a webcam.