Fluorescent tube or spiral bulb ? What is the K number, or colour temperature, it should say on the bulb or the package it came in what that is. For a cool white, it would be around 3200 K, which is not enough light for most plants in water. Daylight is the preferred one, 6500 K, [ Kelvin]. Also, how far away from the surface of the water is the bulb ? Out of curiosity, how far away from the light was the moss ?
I first tried growing it in a 29G, which was in front of a south facing window with an overhanging balcony above it. I had 3 spiral fluorescent bulbs for lighting, 23W each, 6500 K, and the only place in the tank that moss grew nicely was on the back wall, nearest the window.
The lights would have been approximately 22 inches above the moss I tied to rocks on the bottom, less for moss I tied to wood higher up, but most of the moss just did not do particularly well. At the time I didn't use Excel or fertilize, I wanted low tech, but for that java moss, it didn't work out very well at all.
I'm not sure why, because it is supposed to be a low light, low maintenance plant, but sometimes it appears to need quite a bit more attention than it's supposed to.