Many take spiralina capsules claiming that is natures complete food how safe is it?

Caveat emptor: It's dried pond scum -- photosynthetic bacteria. The nutrients that it contains are simply those contained in all living things. It may be a future food to be grown by space explorers, but there's zero real evidence that it's any better than whatever you like. Be sure you get yours from somebody reputable. Why not get with a local evidence-based holistic practitioner for guidance instead?

See details: It is really difficult to answer. This is a blue-green algae and the safety cannot be adequately documented. If it is free of contaminants it is possibly safe. If it contains contaminants known as microcystins it can cause liver disease, heart disease, shock and death. Be very careful. I would stay away from this substance.