"green" or natural implies porous natural stone, inherently need frequent sealer to be applied to prevent stains and midew/
"green" applied to a product automatically means $$$$$ because those wacko fringe people are greedy capitalists like the rest of us!
"natural" stone today is largely granite mined in 3rd world countries by slave and child labor.
A recommendation for a long lasting, relatively easy to maintain shower: glazed ceramic tile. 4" tiles with very tiny grout lines. VERY good ventilation in the bathroom area will do more for mildew prevention than anything else you could do.