It is motionless. Because all of the forces on the cup are in balance. Gravity is a force pulling down on the cup, and the desktop is a force pushing up on it. Gravity would pull it downward. There would be no desk top to push up on it.
Gravity is an unbalanced force on the cup.
Newton understood when all existing forces on an object are in balance, the object will remain motionless, or inert. The First Law of Motion is about inertia. If I apply an unbalanced force that overcomes the inertia of the cup, it moves. Here is an activity to test inertia. Find a block of wood and tear a strip of newspaper that is as wide as the block and three times as long.