Is the tissue just fading from green to brown? Or is there something on the coral that's brown? Brown Jelly disease will look like a brown slime covering the coral.
If the tissue is just changing color, and if this is a new coral to your tank, it could just be adjusting to your lighting. Most corals will shift colors somewhat as they become accustomed to a new tank. (Or maybe it's not happy with high nitrates? Weren't you saying you were at 40-60ppm in that other thread?)