It would be. Cities and urban areas aren't the problem. Suburbs, with 20+ Minute commutes, on hot swollen rivers of concrete and asphalt flowing from them, with every individual in their own metal/polymer box burning hydrocarbons is the bigger problem. Cities might be a solution.

Conversely these algae tanks can go lots of places a tree wouldn't be practical. They'll never need to be trimmed out of power lines etc. Or tear up sidewalks, streets or foundations. That's not to say we shouldn't have trees. Just more green overall.