Why Did My Live Rock turn Brown?

[R34dawn]

If you don't have any corals I would first do a water change trying to get must of the dusty stuff, dont feed the fish more than what they can eat within 5 min. or so and reduce the amount of hours you keep the lights on, that's if you dont have any corals that might need
significant lighting. may also ad some concentrated bacteria like cycle or niteout

[spiralout]

Is it possible to post a few pictures? Easier to diagnose with a visual. XD

Since your tank and rock are still young, it may be as simple as the process of your rock maturing. Do you have sufficient water flow to blow the waste from your aquascape?

spiral