Ideally, a red bush such as AR mini will look best at that spot, but it won’t grow under low light level at great depth. You already have a nice large leaf Buce on the right. Have you considered adding a small leaf Buce at the spot. I have a Red Mini Buce at the substrate level and it has vibrant color but grows at snail pace. The light green color of your Lobella actually looks good at that spot. If you don’t like it in the substrate, you can pot it in a small hydroponic pot and stick into the hole,. If it doesn’t stick you can superglue the plastic onto the rock. All my rosette and stem plants are planted in hydroponic pots and hidden behind rock, and the black plastic blends in easily.