Nothing special happens but it is a pretty and also weird site! As you said, it is a
Munchlax-shaped Rock (Pokemon Rock). If you go south and down the stairs then head west, let a Pokemon use Rock Climb and then enter the house in front of you. Talk to the scientist
in there and he will ask you for a Pokemon with a certain nature... Bring a Pokemon with the nature he asked for and in return, he gives you a Poketch App for the Poketch (Pokemon Watch)!
Hope this helps...