Of course, human population growth has always been a major problem, still is a major problem and (most probably) will continue to be a problem for a long time. But while we seet to address the issue of population as a solution to pollution, this appears to be "work-in-progress", and always will be. So seeking other solutions while we work on the population issue is very important.
I agree with you though that the side effects of using technologically modified algae needs to be looked into, as it could turn out to cause more problems in other eco-spaces.
Just a quick thought too: what about using certain types of biochar in agricultural production? Studies suggest that some biochar made of particular feedstock can potentially increase agricultural yield, yet reduce the amount of CO2 emitted to the atmosphere.