I can't really explain it in technical detail. The vitamins and stuff from spirulina (and chlorella) will go everywhere. The part of spirulina which attaches itself to mercury only has it's effect in the digestive system. You need to get the mercury from out of the places where it's stuck and into your digestive system in order for spirulina to have an appreciable chelating effect.
Otherwise, we could just take spirulina for a while and we would be free of mercury. It just doesn't work that way. We need to take something which has a chelating effect throughout the body (ALA, for example) in order to get the mercury out.