That is strong lighting, you will need to add CO2 and fertilizers or you will certainly have algea problems. How long has this tank been running with this lighting?
But for your question, this really seems like a nutrient deficiency. Probably potassium. Plants need macro (Nitrogen, Phosphorous and Potassium) and micro (calcium, magnesium, and other trace elements) in order to thrive. And carbon. Many easy plants can do well even without fertilizers, but most need them. Especially with strong lighting.
With low lights, you could dose only potassium, it is the only macro element that is not introduced with food and fish waste.
Excel is a source of liquid carbon, it compensates a bit for CO2, it is maybe 20% as effective. Which is not enough for the lights you have.
So quick advice, since I don't think you want to get into CO2 : lower the strength of you lighting, dose Flourish Potassium, and continue dosing Excel.
And to save some money, you can make your own Flourish Potassium for 1/20 the price. Order Potassium sulfate from aquariumfertilizer, then mix 1.4tbsp with 500ml of water and you get the exact same thing.
Or even better, dose full fertilizers by following the Estimative Index, do a little search on it and you should find pleinty of info.