What did I do wrong?

A question from a fellow grower:

However I have a small problem. Kindly look at the photos attached and please advise what the problem may be.(over watering, not enough feed, light…?)

Is it powdery? Does it rub off?

Also it could most likely a pH related issue. What is the pH of the water you give it, the pH of the nutrient mix after it has been mixed with the water, and finally what is the pH in the soil?