The secret to the moon door next to the weeping waterfall in Fez

By turning all of the mini-skulls (hold X) so all of them are facing away from you at the same time, so will the giant skull, revealing the back of it's head.

These are decorative Easter eggs, not a puzzle. Your map node is white most likely because you have another room to find. (Perhaps this one.)

The concentric squares have no bearing on the game or its completion. There are many of them scattered around with different numbers of rings: in Gomez's village (1), atop the broken arch stage (2), atop the bell stage (3), on the 4-cube door's floor (5), atop the waterfall stage (6), on the 16-cube door's temple-under-construction ledge (7), and on the same ledge in the temple warp zone (8), for example.