I want to have the glmatrix xscreensaver running as my desktop background, which works well on pure openbox, but in xfce I need to kill whatever is is running the root window (as far as I understand). What process is this in xfce?
The bit you are looking for is
You should be able to disable it in the session manager (
Go to Settings > Window Manager Tweaks and there the last tab is Compositor. Uncheck it.