Algin is a polysaccharide derived from brown seaweeds. Algin is present as a mixed salt (sodium, potassium, calcium, magnesium) of alginic acid. Therefore, there is some natural sodium in sodium alginate.

However, since mercury indirectly leads to high blood pressure (see explanation as to how below), I would be more concerned with getting rid of the mercury in my body.

Mercury reduces or inhibits the body's ability to create methylcobalamin (methyl B12), which in turn (through a multi-step process), leads to elevated levels of homocysteine, which is known to cause high blood pressure.

I have not found any information that other "salts" of alginic acid have the same effect on heavy metals like mercury that sodium alginate does.