What is this mono process running at startup?

As pointed out in the answer by Thomas, it's easy enough to stop or disable this server, but you may want to consider whether you actually might want it running before you do. I found this question while researching this exact same concern on my own Kubuntu 18.04 LTS machine, and this helped me dig deeper into what it actually was. Turns out it's used to serve monodoc-http, as I was able to determine by exploring the content of files found in /etc/xsp4 and /usr/share/monodoc/web.

If you installed MonoDevelop or MonoDoc or anything related to developing code in Mono, then that's where it came from. If you did that on purpose because you're learning or coding in Mono, then you may actually want that service running. If you have no use for MonoDoc services, you can easily disable them on flavors of Ubuntu (and other distros which use systemd) using systemctl.

• sudo systemctl stop mono-xsp4.service will stop the service
  (but won't disable it, so it will run again after next reboot).
• sudo systemctl disable mono-xsp4.service will disable the service
  (but will not stop it if it is running currently).

You'll need to run both commands if you wish to actually stop the service now, and disable the service so that it's not running after the next reboot. If you know for certain that you have no use at all for MonoDoc and related services, you can also search for them in your package manager and uninstall them entirely.

The mono-xsp4 is shipped with a SysV init script at /etc/init.d/mono-xsp4.

To disable the automatic startup run the following command.

update-rc.d mono-xsp4 disable