How can green algae be both multicellular and unicellular? What are the cases in which it can be both uni and multi cellular?
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
Guiry, J Phycol 2012; 48, 1057–63