Okay you might want to take a look at the system-wide xfce keybindings at:-
xed /etc/xdg/xfce4/xfconf/xfce-perchannel-xml/xfce4-keyboard-shortcuts.xml
and if exists the user specific copy at:-
xed ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-keyboard-shortcuts.xml
both of which contain the
custom properties keybindings .. such as:-
<property name="<Alt>F8" type="string" value="resize_window_key"/>
which you might want to change to say
<property name="<Super>F8" type="string" value="resize_window_key"/>
or just disable it with
<property name="<Alt>F8" type="empty"/>