sorry, what i wanted to say
as to hippos, many are known to be shy, they are known for hiding in the rock even ater being in the tank for a while. thye are even noted for lying upside down and sideways wedged under or in a rock. In such a position that we would swear is not right and would swear that it could even be dead.
if it is breathing and eating and looks healthy, dont worry too much about it.