Why would you call Codium a unicellular organism?
Codium is a type of seaweed that is considered a unicellular organism. This is because even though they divide nuclei to grow larger, they do not separate into smaller cells by building cell walls.
Virus is considered as a living organism since it doesn't have what we call a well-defined cell. It has a protein coat and genetic material only. It is a matter of controversy whether to classify viruses as living or non-living. They lack all other cell components. Hence, it cannot be classified as unicellular or multicellular.
My best guess is when anything reproduces mutations may occur in the DNA. Some mutation happened in the DNA and the unicellular organism to start replicating itself. (We see call this mitosis) The mutation turned out to be a good thing and helped the organism survive, later generations through more mutations started orientating the cells into a specific shape... making a whip like tail so it could propel itself forward and so on and so…