The YouTube shots are confusing, as sometimes the subject has stubble, rather than a five-o'clock shadow or a goatee beard, other times nowt at all.

Even as a lateral, non Argentinian thinker or tinker, I doubt the phrase is meant figuratively or metaphorically - whereas, on Cuba, bigote de sombra is a 'prison beard'.....

BTW, in the UK it's just stubble, rather than a stubble beard, as in the example sentence.

interpretando que alude a la forma, al trazo peculiar y fino del bigote (cuando no se ha afeitado)

I would also use "shadow", even though it doesn't go exactly with the definition of the word, but it is in a figurative meaning, because it is like the shadow of a mustache that starts to grow.

Bumfluff 'tache. It really is a thing.

http://www.collinsdictionary.com/dictionary/english/bumfluff

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Note added at 18 hrs (2023-01-23 12:59:56 GMT)
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Or, to avoid the perceived vulgarity of "bum-", you might try something like "a hint of a moustache" (38,000 + hits in an online search).
"A Praetorian ... with, rather interestingly and unusually, a hint of a moustache! ...
"...a hint of a moustache, his hat set at a rakish angle"
http://www.jstor.org/stable/40480062

http://www.the-humble-essayist.com/the-other-steve-harvey

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Note added at 18 hrs (2023-01-23 13:01:26 GMT)
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"He ...looked like a young man, a hint of a moustache..."
http://roleofmusicingreekmythology.weebly.com/hermes.html

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Note added at 1 day 14 hrs (2023-01-24 08:40:49 GMT)
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NB: I think my suggestions so far are valid. The first, "bumfluff" is more irreverent, whereas my second offering, "hint", is more elegant.