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-genmon-plugin". (same bug reported on launchpad: https://bugs.launchpad.net/ubuntu/+source/xfce4-genmon-plugin/+bug/636548 )
It would probably be better to use an existing icon, such as 'utilities-system-monitor', which is (kind of) more informative about what the plugin can do, and it doesn't break existing icon themes.
Fixed in Git: http://git.xfce.org/panel-plugins/xfce4-genmon-plugin/commit/?id=2fd3d192741de9eb97a84e8fae7db49a2dc4b0e1
Fixed in 3.1.1.
Er, 3.3.1.