How can green algae be both multicellular and unicellular?

Because Algae are represented across four kingdoms, namely Bacteria (unicellular prokaryotes), Plantae (multicellular eukaryotes), Chromista (multicellular eukaryotes), and Protozoa (single-celled eukaryotes).

For example, I. Chlorophyta is unicellular:

^{I. Chlorophyta. Source: University of Wisconsin}

While kelp can become huge and can form underwater forests:

^{Kelp. Source: Nature}

Reference
Guiry, J Phycol 2012; 48, 1057–63