the respective .desktop file in /usr/share/xfce4/panel-plugins uses "xfce-mouse" in its "Icon=" line. please replace that by something more meaningful (and theme-ingful ;) ) like "xfce4-netload-plugin". (same bug reported on launchpad: https://bugs.launchpad.net/ubuntu/+source/xfce4-netload-plugin/+bug/636535 )
It would probably be better to use an existing icon, such as 'gtk-network', which is (kind of) more informative about what the plugin can do, and it doesn't break existing icon themes.
*** This bug has been marked as a duplicate of bug 6864 ***