'Aliens' visiting Earth would immediately categorize us into groups very crudely resembling the groupings we use today, because our visible characteristics are the most immediately obvious artifacts of our existence.
(Edit: when I say 'immediately' I'm indicating this would be an obvious, first order thing to do from the first pictures they have of us, not an 'Enlightened Alien' scientific form of categorization).
If all they had were 'pictures' of us, the race categories we use would be the obvious grouping, or something resembling that.
They would see that most of the people in Sub Saharan Africa looked quite different from those in East Asia. (And difference between Sub-Saharan Africans and East Asians is more than 'melanin').
There's a 'continuum' between every biological grouping, that doesn't mean those categories don't exist. It just means we're going to argue a lot about where and how to draw the lines.
Race as a 'Social Construct' relates to all of the other attributes that we associate with race, and individual lived experiences due to how they are perceived etc..
To your point, Aliens wouldn't immediately pick up on the 'Social Construct' bit, at least not right away and so they wouldn't have the prejudices that we do, but if they could only observe from afar, they would see exactly what we see, and visual distinctions would be the 'first order of separation' even if it was, after further understanding (i.e. genetics) a less important distinction as you hint.
Edit: someone provided this like I'd like to also include it [1] which illustrates some of the current debate over the notion of race, and that it's clearly politicized.
[1] https://en.wikipedia.org/wiki/Race_(human_categorization)