[e-dt]

'masculus' does not seem to come from the Sanskrit 'marya' (both may have come from the same root), and 'marya' does not seem to mean mortal but rather young man. 'martya' (from a different root) does mean mortal, though, which is where you may have become confused.

[mdp2021]

> both may have come from the same root

But they seem to have the same meaning. The identified root seems to be "mṛ", that of mortality, and they may (/may/, with relevance to the content this paragraph) come from it; that one term in Sanskrit in inherited by Greek and then Latin is the «come from» I meant.

> and 'marya' does not seem to mean mortal

So where does it come from? According to some authors, 'marya' comes from "mṛ" just like 'martya', which you state has a different root. Edit: disambiguation: of course 'marya' is used for "young man". But it seems to mean "a "mortal", someone ready (initiated, young man, no more a child) to develop his defined (limited) path of life".