Green Hydra turning White

Cholorohydra is a green coloured hydra, which has Chlorella an algae inside its cells, it is called to be having a symbiotic relation with hydra. Students are designing an experiment to check if Hydra grows the most in presence of light and the least in absence of light.
There is an observation that green hydra are turning into white in dark as there is no photosynthesis in the dark conditions.
How can we prove it?

The experimental design to check what is the most suitable light conditions for hydra growth