You can try adding more surface disturbance. Aim a powerhead or other flow source more towards the surface so you get light rippling. That will cause more of the CO2 to be lost, and reduce the ppm somewhat. I doubt that a drop from around 40 ppm to around 30 ppm will cause an algae bloom.

Just to be sure about your conditions: are you using 4 dKH distilled or deionized water in the drop checker and not tank or tap water?