'Building socialism' and 'actually existing socialism' are pretty much synonyms, because the terms for the transition stage between capitalism and communism are vaguely defined.

Some call the end point of a "classless, moneyless society" communism, some call it socialism - and some call the transition period, where a society still has features of capitalism alongside features of communism, socialism.

Ultimately it comes down to: who holds political power in a given society, how strong is their grip on it, and how forcefully are they pushing in the direction of communism?

For myself, I would use the word 'socialist' for any country that's somewhere along the transition stage, with whatever features are peculiar to that particular country's history, as long as it's controlled by a Dictatorship of the Proletariat led by committed communists.

So for instance, I would call the USSR, China, the DPRK, Cuba etc. "Actually Existing Socialism" whereas countries with 'socialist policies' (basically, pro-welfare and pro-development) but no DOTP like Venezuela are less clear. Conversely, imperialist nations with big welfare states, like Norway for example, despite being called 'socialist' all the time in the capitalist media are very certainly not.