What do you call the frequent conjunction {juxtaposition} of two words?

Solution 1:

The obvious choice is to call it an expression:

A word or phrase, especially an idiomatic one, used to convey an idea.
_{Oxford Dictionaries}

(In fact, you've already used this in the question.)

Alternatively, you can call it a phrase.

Solution 2:

A word I actually only started using after following this site is collocation:

[Oxford]

1 Linguistics
The habitual juxtaposition of a particular word with another word or words with a frequency greater than chance.
‘the words have a similar range of collocation’

1.1 count noun A pair or group of words that are habitually juxtaposed.
‘‘strong tea’ and ‘heavy drinker’ are typical English collocations’

2 The action of placing things side by side or in position.
‘the collocation of the two pieces’

There are actually dictionaries, such as OzDictionary, that are devoted to collocations.

(Ironically enough in terms of this answer, that site doesn't list "imposter syndrome" as a collocation—the closest it located for me was "Californian syndrome." However, any such resource is only going to list common collocations, not ones that you might be personally noticing.)