Dwarf Hairgrass might work if your light isn't too strong, otherwise it will eventually cover the substrate. Also, small crytocorynes (willisii, lucens, parva...) would work nicely and leave some substrate visible.
There is also a link to it in the top left of your screen in a gold menubar. Once you're in the PlantFinder, look to the right and you'll see an option to search by placement. Select Foreground and it'll give you several plants that will work.