Most of vegetables and fruits with green skin have more nutrients concentrated in their skin than flesh of the fruit. Yes, even watermelon skin is edible and full of nutrients. His giving it a religious touch to it might be due to some hadees or something that narrates the benefits of eating green skin of cucumber. I do not know if such hadees or narration exists as I'm no scholar.
The thing I do not understand, why people are so harsh on him even if he motivates people to eat it just because of its green color!!!!!!!!
The whole idea is; not to waste it when you can use it. Isn't it??????