Sorry no one has answered this yet!
The 11 watt should be fine, what you're looking for is a light that goes all the way to the bottom of the tank, not just midway down. But anubias and most mosses are low light plants so you should be fine either way as long as they get about 8 hours of light a day.
Although I'm not very experienced in moss other than Flame moss and Java moss, I believe Willow moss is very similar to Java moss and thus should have similar requirements.
Rembrandt II, Suiché
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen
Gargoyle: Cloudjumper and Toothless