Java moss will grow better with some current/movement of water in the tank. Adding a powerhead will probably make it grow faster. Also spreading it out and tieing it in individual strands will help it grow faster as well. The key is to expose as much of it as possible to light. Having it in a mass doesn't allow the bottom to get much light at all. How much lighting is on the tank? Java moss doesn't need much light to grow, though it does grow faster with higher lighting and it turns bright green as well with increased light. The main reason that I'm asking about lighting is because it doesn't appear from your pic that you have high lighting, and without that the CO2 won't do much good for the plants. They need lots of light to use lots of CO2.