"Orders of magnitude higher" levels of mercury concentration sounds bad, but is it high enough to be concerning? How much of that mercury would be left in a tea bag?
Most likely more mercury than is found in the plastic alternative. The mercury may be bound somehow to the seaweed in a way that prevents it from being absorbed when the seaweed is consumed although steeping it in boiling water may leech it out. Ethyl mercury contaminated water is something to be avoided entirely.
It's made with processed Seaweed, so I'd think the end product would be at least mostly free of these contaminants.
I would expect the opposite: this product needs to be cheap to have a shot at supplanting plastic; extra process to remove all such contamination would work against it
I think the process would be low-cost (reverse osmosis maybe?). Filtration generally seems pretty low-cost all around - the LifeStraw is $12.99 and can make toilet water drinkable.