Live rock harbors beneficial bacteria that helps with the natural filtration of the water and chemicals like the ammonia and etc. The mechanical filtration/skimmer does its thing to clean also by filtering the undissolved organics.
Depending on whether you're doing fish only or reef & fish, the recommended amount of LR
is anywhere from 1lb to 2 lbs per gallon of water.
Have you tested your phosphate level? Also, the tank isn't anywhere near a window with any amounts of sunlight peeking thru, is it?