If your photo was taken mid-way through the photoperiod, then it will be a CO2 issue. You are able to reduce your demand for CO2 by limiting other nutrients; since you are dosing EI, you are making it so that the only limiting nutrient (in theory) is CO2 - the hardest to dissolve and distribute.
The drop checker has a 2 hour ish delay; if you come and check it every 30 minutes after CO2 turns on, you will see it change color throughout the day.
Aim for it to turn lime greenish ~30 ppm of CO2 about 2 hours after lights on, then watch your tank. I wouldn't worry about lights on color as the rate that it changes color is based on your injection rate which is based on your surface agitation - that differs by each tank.
The other edge of the CO2 sword is the flow/distribution of the bubbles/dissolved CO2. So watch your bubbles. If your diffuser has bubbles going straight up to the top and none are being moved around the tank, then you need to work on your placement. Do not stress if some get to the top - we can go crazy trying to manipulate gases. The other option is to place your diffuser in your filter intake and have the filter itself eat the bubbles and distribute in the path of your lilly pipe.
Once it looks "like bubbles are moving around the tank" (or you have most of the bubbles going into your intake" and you have the DC lime greenish by 2 hours after lights on, you are good. If your DC is hitting Lime green, just not at lights on, then turn your CO2 on earlier to hit that lime color by lights on.
Oh yes, once you get CO2 set and while you are setting it, you need to clean - clean intensely - rub the leaves, remove the gunk, let those dead leaves float to the top and skim them away - all you want left is healthy leaves (I mean leave the healthiest ... if they are all unhealthy, then you need to keep some)!
Ultimately, watch new growth - if new growth is algae free, then you are on the right track.