Please see your sentence "If you are talking about an adolescent who is just starting to grow facial hair". Related to this could you please tell me whether we can write "If you are talking about an adolescent whose facial hair is growing"?If you are talking about an adult male who normally shaves, then if it is only a few days old, the best description is probably "unshaven". The short hairs are "stubble". If you are talking about an adolescent who is just starting to grow facial hair and haven't started shaving, there are a variety of expressions depending on how rude you want to be ("bumfluff" comes immediately to my mind); this is far thinner than an adult's bristles.
Perhaps you could clarify your question. Is the man with the moustache deliberately not shaving the upper lip and still shaving the rest of his facial hair? If so, I'd just say "He's growing a new moustache" or "He's got a new moustache".What would we call the moustache that is normal in size but not dense as in a few days after shaving?