Land.

50 trees take a lot of space. The idea is to put these boxes where trees can't fit.

Note that these algae are not taking the room of 50 trees, that's really not the way to look at it. We want more trees inside cities, and whether or not this happens is totally decorrelated to whether or not we see these algae boxes or advertisement in the bus stops.

how much more expensive is manufacturing and maintaining them than planting and watering a tree is?

I don't think it is hard to imagine that a box of algae takes less maintenance than a tree inside a city. Typically you don't plant a seed in a city to grow a tree. You grow very specific resistant species in a tree farm and then transplant them once they are tall enough in a city. This is a long and expensive progress. Trees require maintenance: they need to be trimmed, healed, they may have fallen leaves that need cleaning, they may need watering when it is too dry, they need removal if they are too damage.

I think a tree has probably a similar level of need than these boxes, so 50 trees will largely exceed that.