With discus I suggest you do not plant the foreground. Having a planted
foreground will cause issues with food getting trapped in there (assuming
you're feeding them well) and will not allow the dicus to blow in the sand to
kick the food up.
However, if you really want something in there you can
use Echinodorus tennelus var. micro
Hairgrass
Marselia crenata or Marselia drumondi