It is hard to convey how green a mango needs to be for this salad, but I have never come across one that was too green — provided it is fully grown. On the other hand, I have bought mangoes that appeared green and felt firm (even in Phnom Penh markets) but which, on cutting, revealed flesh that was already sweet, orange and upon grating turned soft and mushy. Choose a fruit that you think is too hard and all will be well. In Cambodia, a special wavy-edged peeler you can find in the markets does an excellent job of grating green mango. If these are not available, grate on the coarse side of your cheese grater.