Hey dude,
Yea initially it will take a couple months to really thick out. This is a really fast growing plant once it's established. Instead of a rock though i would suggest a big piece of driftwood and get more java moss if you can manage and spread it out on the java moss, it'll take a couple months to root itself in and start to grow. Java moss can grow with just water and light ti's a great hardy plant. I would highly suggest wood to grow on instead of a rock, i think the end result would be alot nicer aswell