They seem to be different: a Code Name can be anything, but a Character Name Alias must be the name of a fictional character. For example, if I use the name "Red Troper", that is a Code Name; but if I use the name "Bugs Bunny", that is a Character Name Alias.
True; the Character Name Alias does however make a terrible job explaining that in its name (I'd think Borrowed Name As Alias or similar would be far more clearer).
Character Name Alias is however clearly a subtrope of Code Name.
Would Line-of-Sight Name, Sdrawkcab Alias and Sue Donym also be subtropes of Code Name?
Guess so.
Would be nice to have some sort of metadata way of adding and view trope relationships, apart from the parent/child relation we already have.
I found the following two tropes during an YKTTW discussion: Code Name and Character Name Alias.
From my reading of the descriptions, I see no real difference between the tropes. Code Name appear to be more established (more wicks and inbounds), and with a more thought-through description.