Usually 乜野XX is used to ask for clarification when someone mentions XX. Thus, the following conversation makes sense:
A: 兩種花生糖唔同架 (The two types of peanut candies are different.)
B: 乜嘢唔同？ (What do you mean "different"?)
However, if you are just browsing and the vendor does not say anything, it may be awkward to suddenly ask 乜嘢唔同.
A: 兩種花生糖唔同架 (The two types of peanut candies are different.)
B: 乜嘢唔同？ (What do you mean "different"?)
However, if you are just browsing and the vendor does not say anything, it may be awkward to suddenly ask 乜嘢唔同.