Metformin is a prescription medicine with established uses in diabetes care. Spirulina has not been shown to be equivalent to metformin and should not be presented as a replacement for it.
Reviews of spirulina trials report possible changes in some glucose and lipid measurements, but the studies are heterogeneous and do not establish equivalence to prescription treatment. Evidence concerning isolated phycocyanin is also largely based on animal and cell studies rather than human clinical trials.
Do not stop or change metformin without speaking to the clinician who prescribed it. Anyone monitoring blood glucose should discuss dietary supplements with their healthcare professional.