If your pleco is turning white, it could be a sign of stress or disease. Stress can occur from too much light in the tank, sudden changes in water parameters, or overcrowding. It's important to check for any signs of illness and make sure that all water conditions are good for the fish before assuming it is just a case of stress. Other possible causes could include nutritional deficiencies and even parasites like Ichthyophthirius multifiliis (ICH). If you suspect either one of these issues, consult with an experienced aquarist to determine if treatment is necessary. Additionally, be sure to keep up regular aquarium maintenance such as weekly partial water changes and cleaning out debris from the substrate.