Zooxanthellae that live in corals are brownish in color, and when they become more prominant in the tissue the coral takes on this color, hence "browning out". Zooxanthellae are flagellate protozoa that produce carbon compunds for the coral to feed off of via photosynthesis. Generally speaking, when the coral isn't receiving enough light, they produce more zooxanthellae in order to prodcue more food for the coral to feed off of and "be happy". Once they're recieving enough light and are "happy" they shed unecessary zooxanthellae and begin to color up. On the flip side, bleaching is caused by too much light... the coral sheds lots of zooxanthellae and turns light colored or white.
With that said nutrients do play a role in corals browning out as well. Make sure to get all your parameters in line and stable.
That coral appears to be a maricultured specimen, and it's not uncommon for them to brown out intially, but they'll eventually color up if in good conditions.