You’re clearly alluding to the words in The German Ideology about communism being the real movement which abolishes the present state of things. And yet in the same work, and in several others, Marx and Engels also do talk about “communist society” and give some rough descriptions of how it would operate. The notion that communism is pure negativity and that it “goes away when capitalism goes away” (or that "the idea of socialism as the achievement of minimum demands, that leads to communism as the realization of maximum demands, is not a Marxist communist theory of change, it is a social democratic one") is something you would have to take up with a lot of works, again, but this is most clearly put down in Critique of the Gotha Programme.

And I don’t think Marx would go so far as to say “communism exists now within the working class,” because there is no world-historic struggle by the collective proletariat to the present state of things; struggle on the individual level is not communism, as is also made clear in TGI, nor would communism exist within the working class semantically regardless, as it is the [which seizes upon the immanent negativity in capitalism and reorganizes production, thereby upheaving capitalist relations of production], it is not some rebellious spirit people come to possess.