Not a malapropism but a ... what?

As suggested, solecism may be an apt term, but barbarism could be even closer, seeing that the example given in the question shows a fault involving a single word. This summary of a section from the Roman grammarian Donatus's Ars Grammatica neatly distinguishes the two:

A barbarism is defined as a fault of a single part of speech, generally a word form...while a solecism is a fault of a part of speech used in connection with other linguistic units.

from Ars Poetriae: Rhetorical and Grammatical Invention at the Margin of Literacy By William Michael Purcell

The patronizing, puritanical air of barbarism is admittedly a long way from the comical nature of malapropism, and Ngram Views shows its (as well as solecism's) use steadily declining to almost nil today. Yet modern dictionaries still maintain its meaning (though always secondary to the sense of "barbaric act") much the way it was described in the 4th C.E. century by Donatus:

barbarism

3. An error in language use within a single word, such as a mispronunciation.

Wiktionary

2a. The use of words, forms, or expressions considered incorrect or unacceptable. b. A specific word, form, or expression so used.

(AHCD, 4th Edition)

You might be searching for catachresis, which is defined by google as "the use of a word in a way that is not correct, for example, the use of mitigate for militate." While mitigate and militate sound similar, this is not characteristic of catachresis. Catachresis is more general, I believe, and applies to confusions of semantically similar words. Such an error could be called a catachresism.

It is true that the expressions have unique set phrase definitions, so each would be preferable for its specific use:

• Common knowledge

Something known by most people:
it’s common knowledge that no one has yet found a cure for cancer
_ODO

• General knowledge

noun

[MASS NOUN] Knowledge of a broad range of facts about various subjects:
this quiz tests your general knowledge

[AS MODIFIER]:
a round of general knowledge questions
_ODO

But it would be quite parochial to impose the set phrase interpretation. The adjectival elements of each expression are similar in their meaning and both can be combined with the noun knowledge with insignificant shift in meaning:

Common

adjective (commoner, commonest)

1.0 Occurring, found, or done often; prevalent:
salt and pepper are the two most common seasonings
common misspellings
it’s common for a woman to be depressed after giving birth

1.3 Ordinary; of ordinary qualities; without special rank or position:
the dwellings of common people
a common soldier

1.4 (Of a quality) of a sort or level to be generally expected:
common decency

1.5 Of the most familiar type:
the common or vernacular name

2.0 Shared by, coming from, or done by two or more people, groups, or things:
the two republics' common border problems
common to both communities

2.1 Belonging to or involving the whole of a community or the public at large:
common land
_{ODO Emphasis added}

Common knowledge yields the reasonable meaning:
Ordinary knowledge belonging to the whole community

vs.

General knowledge yields the reasonable meaning:
Normal knowledge affecting all people

General

adjective

1.0 Affecting or concerning all or most people or things; widespread:
books of general interest
the general opinion was that prices would fall

1.1 Not specialized or limited in range of subject, application, activity, etc.
brush up on your general knowledge

1.2 (Of a rule, principle, etc.) true for all or most cases.

1.3 Normal or usual:
it is not general practice to confirm or deny such reports
_{ODO Emphasis added}

Alternative:

adjective

1 [ATTRIBUTIVE] (Of one or more things) available as another possibility or choice:
the various alternative methods for resolving disputes
_ODO

Common knowledge is about half again more common than general knowledge in the corpus, but they have been roughly tracking together for about 75 years:

Conclusion

It is certainly appropriate to interpret common knowledge and general knowledge as set phrases, but neither phrase is dominant enough to exclude the simple combination of the words. Therefor, in appropriate contexts general knowledge remains an acceptable alternative for common knowledge.