Excel is a liquid that has the same effects to your plants as CO2 but its also a strong cleaning agent, dropping water level, exposing the algae and spraying Excel mixed with water 1:20 ratio over it and leaving it for few mins before refilling will kill it. over a few days it will turn red and die off before vanishing. In future just do all water changes after lights out or get your water prepared in a container with the lid off for a few days before your water change so the CO2 in the tap water gasses off before it goes in the tank. That way the small fluctuations in CO2 wont happen and you want get the BBA back.