Spirulina is the lowest form of algae like Nostoc and they have blue-green color. Its rich in vitamins and since they photosynthesise, they are classified under plant kingdom. So dont worry you are not eating anything nonvegetarian. Spirulina is 100% vegetarian that is also 100% healthy in its nutrient. But Its taste is awful when taken as powder, that I have tried. So if you are taking as capsules, you are better off in one gulp.

The following link gives general info about Spirulina
http://www.naturalways.com/spirul1.htm