I would think about what you are adding to your aquarium in terms of bottled chemicals.
Excel is not a fertilizer, more an algaecide that will allow the plants better access to what little CO2 there is in the water, it might also cause plants to melt.
If you are using CO2, then there is no need, is you are not, I still wouldn't necessarily use Excel on a routing basis (or any of the other 'liquid carbon' supplements). I f you need to fertilize I would consider something like Thrive (Get it from NilocG.com or amazon or whatever)
But if the grass grows, it should absolutely cover all of the substrate except where stopped from doing so. I think it would do better with supplemental CO2, and I'm not entirely sure about the kit light that comes with the Flex.
But see what happens, know what deficiencies will look like.
Aesthetically, I would have tried to have the rock taller, it's likely to get lost in the aquascape unless it's many times bolder (Excuse the pun), I'd like to see the rock to the top 10% of the aquarium, if not out of the tank