Doubt regarding a simple electromechanical system
In fact decrease in air gap should lead to increase in permeance so
  that energy stored should decrease


With AC excitation, if the core permeance increases then the winding inductance also increases. This means that the impedance to the AC excitation is greater and this reduces current. Given that a linear increase in inductance produces a linear fall in excitation current, then the stored energy in the magnetic field is reduced because of: -

W = \$\dfrac{LI^2}{2}\$

However, for a DC excitation, it can be assumed that the coil current is held constant and therefore, because inductance increases as the movable member closes the gap, magnetic energy stored increases with L increasing.