Since current= Q/t
and V=E/Q
Taking in consideration that the voltage and the time remain constant, for the current to decrease it means that the number number of coulombs decreases.
Meaning that there will be smaller number of electrons to form annihilate holes in the P-N junction, emitting less photons and therefore decreasing the brightness of the LED.
Note that since the energy of the photon does not decrease since it only varies when the wavelength and frequency changes, it simply decreases the amount of photons emitted.