It is working properly
From the manual
Accessories: Photo-electric cell 7094.05
By placing the photo-electric cell in the illuminated
area of the aquarium lamp, the pulse operation of the
pump is also interrupted when the lamp is switched
off. The pump will then continue to operation at the
performance set on the ”Power 2” knob (7). LED
(5) will light up red. In the morning, when the light is
switched on again, the selected pulse operation of
the pump will start again