I get this question often, I’ve answered in more detail before, you can check the blogs archives for past responses.
To understand the difference there’s some things we have to understand about Marx.
First, Marx was not a Marxist, he never applied socialism or communism, he was a philosopher, a researcher, a visionary, an academic and an idealist. Therefore, those who take his ideas must understand that they were not the product of trail and error, or application, but they were rooted in ideas and philosophy.
Marx had the opportunity to study and directly observe capitalism, industrialization, mercantilism, and class differentiation, it’s what he was born into and lived in his entire life, so he had real world experiences and examples of all that and his study, analysis, and projections for capitalism are spot on; even today they stand.
As for what came after capitalism Marx could only speculate, predict, and proposes structures and systems; Marx predicted, accurately the fall of capitalism, and the ongoing crisis and conflicts of capitalism, and he speculated and advocated for Communism, Socialism, and the eventual dissolution of the State and it’s structures.
Marx was right about most the past and present, but he was off in many respects about the future; that’s where Scientific Socialism comes in, it’s not based on idealism, philosophy, and visions; it’s rooted in ecology and fundamental statecraft. It’s a economic system not established to generate profit, or extract wealth from the lands or people’s labor, it’s an economic system structured to meet the needs of citizens and the ecosystems. All economic activity and distribution is tied to and regulated by scientific analysis and research, not ideals, like Capitalism and Communism. Production and manufacturing is not driven by profit, but by need.
Scientific Socialism is not Marxist, it’s informed by Dialectic and Historical Materialism but not exclusively; it’s also rooted in African economic history and practices, Africans were not only the first Socialist, they founded the first markets and economies on the planet; Africans founded long standing and ecological economies.
What separate SS from Marxism and Communism is that it’s not simply an ideology, or vision, it’s rooted in historical economic practices and protocols that are alien to Europe and Europeans in general.