Unlike fish, Algae are basically the bottom of the food chain. Mercury is an issue because it bioaccumulates in higher trophic levels. Because Algae is right at the bottom, it won't collect very much at all.

The issue with marine algae is not mercury, but arsenic. We know inorganic arsenic is toxic, but marine algae accumulate arsenic in organic molecules, which some studies seem to show that is less toxic or even non-toxic. But some effects of arsenic toxicity are long term, like cancer, and apparently it is not a decided issue yet. There has been arsenic in the sea for a long time, and marine creatures of all sorts have evolved to deal with it in various ways.