If you can, power stuff off and check if that web page is still available. Start with any Windows machines. It could be a virtual machine running inside of something else though.
Edit: here’s how to disable that web server https://superuser.com/a/1377078 . I’d do that on any Windows machines as well.
I’d go around unplugging anything that might be connected to the network, like game consoles, smart TV, etc. It’s unlikely, but maybe something has copied that same screen to throw you off.