Ha, ya know, I've wondered the same thing...but frankly, algae has been around longer than humans, so I don't worry to much about it being toxic. To be honest, my 'chids with more algae grow better than the ones without!! I feel like most of the time, its just me (silly human) imposing my aesthetic opinion on a plant that is otherwise completely fabulously happy. So, more to the point: unless we see algae growing up into our orchids, growing on their leaves or otherwise somehow harming them, I think the two can live in harmony in S&H...perhaps even in happiness, IMHO
