Mooning gnome
Mooning gnome is an image, or collection of images, used on Facebook to warn users that a group is without an active admin and therefore has gone downhill and has no quality anymore. In the worst case the group is taken over by scammers and/or spammers. The image is placed ant there is no admin to remove it.
I don't trust this post about a student room, because there was a mooning gnome right above it, in the same group.