I've had the same problem with some of the pillow mosses, although some still grows in well lighted areas of many of my tanks. You can use Java moss, although it is a bit taller and you may need to trim it back a bit, and does best if your substrate is rather wet. One nice ground cover, although it takes a while to establish, is Ficus pumilio var quercus,(miniature oak leaf ficus) or a very similar plant (not sure it isn't the same thing and one has been misnamed,) is Hedera helix "duck foot" that Glasshouse works in Ohio carries under this name. Other Hedera helix called "Duck foot" ivy is much larger and different--not a ground cover at all. Other than that, a layer of leaf litter such as oak leaves or magnolia leaves makes a nice topping, and looks good, when nothing else will grow. Neither of these deteriorates rapidly, but you can just keep adding more if it does. It also provides tannins to the drainage water if you have a false bottom and falls, etc.
-----
Patty
Pahsimeroi, Idaho
D. auratus blue
D. galactonotus pumpkin orange splash back
D. imitator
D. leucomelas
D. pumilio Bastimentos
D. fantasticus
P. terribilis mint and organe
D. reticulatus
D. castaneoticus
D. azureus
P vittatus
P. lugubris