Humans really are weird. Trying to replace a perfectly fine bio-machinery that developed over Thousands of years with their own steel junk. I dont see why anybody would prefer that gadget over a tree.

Can you plant a tree capable of capturing the same amount of CO2 as those algae in that small a space? How about "refilling" the tree if it happens to die?

Society doesn't have to lock itself to a single solution for countless varied problems. If we're talking about a long, empty walkway, or a park, then trees are a great solution. If we're talking about a small space that must be kept free of obstructions, such as a bus stop, then a sack or box of phytoplankton is much better suited.