Start it very low in the tank to make sure it does not go into shock from excess light. After about a week you can move it up about 1/2 way between the light and the sandbed.
Wedged in the rock is fine. Eventually is will grow and attach it self to the rock (but that takes a while, so you can start it low and mive it up a bit later on).
How many hours are you running your lights?
I think you have a metal halide / T5 combination. I would run the T5s for about 12 hours, and the metal halides for about 10 hours max.