There's a thread on another forum about making a riccia or Java-moss carpet. Since you're low light, java moss it is. Get either stiff plastic netting (needlework net works) or thin slates or nylon screen (if you use net or screen, you probably will want to tie some small pieces of slate to it will monofilament to weight it down.) Lay the java moss on it thread by thread, one layer deep. Stretch a hairnet (like the lunch-lady wears) over all of it and pull it tight (make sure it's tight or you'll get a dome). Use light test monofilament if necessary to pull it tight to indented parts of the frame. Lay the slab on an open patch of substrate. The moss will fill in and hide the net, monofilament and frame. Voila! Low light lawn.
Personally, I use slates so that I can prop them up for my loaches to swim under. Looks like crap for about a month or two, then starts looking good almost overnight. With an 18W light in a 10 gallon tank, it should work pretty well. I've got about 60 or 70 watts over a 50 gallon breeder, and it's working OK there, and 130 watts over a 55, and it's going like mad in that one.
Logic is only the beginning of wisdom