Chlorella is not in the same physical category as Spirulina and Blue-green 'algae'.
Chlorella is a single celled green algae.
This means the cells have a defined nucleus. Blue-green algae are grown in lakes and ponds. Problems arise as these growth environments may contain impurities or agricultural run-off. This harvesting method may not be ideal as other matter may be included with the product, such as various species of bacteria that may harbor toxins, or other impurities. The danger arises because there are no guidelines requiring those marketing Spirulina to monitor their products for contamination by potentially toxic cyanobacteria or by cyanobacteria toxins.