What energy store is decreasing while the candle is burning?

The wax is a store of chemical energy. The wax transfers energy to surroundings as light and as heat. The heat energy dissipates to the surroundings.

The chemical energy store is decreasing and is being converted into heat and light.

Chemical energy is converted to heat. The heat from the wick melts the wax which gets absorbed in the wick and then gets burnt (which is really oxidation) to produce heat energy as well as light energy.

The fuel for burning is the wax which is constantly decreasing once it is turned into vapour it combines with the air oxygen to form the flame.