Your cory catfish is turning white because it’s stressed or ill. If you just bought your cory, it’s likely just turning white or pale because it’s adjusting to the new environment. Once your cory has acclimatized properly, its color will turn darker and richer again.
However, if you’ve had your cory catfish for a while and it suddenly turns white, it might be sick. Immediately check your water conditions and see if anything is unusual. If not, check if your cory catfish is experiencing other symptoms.
Color change can also be caused by a sudden diet change, microbial infection, or fungal infection.