Flow, flow, flow.

If you are cleaning rocks once every 2 weeks, it isn't enough to rid you of the problem. Clean them daily even if all you do is go around a blow each rock off with a baster. Each time you clean, you release nutrients in the water from the dieoff, so the less you do a time creates a better balance in the system. I also wouldn't do a larger water change than what is needed to clean the rock (unless you have high nitrates or phosphates), as you are importing new nutrients that it feeds on too each time you do. If you are adding any trace element supplements, stop.

If you take the rocks out to clean, be sure to use your existing tank water to clean and rinse them, then put new water directly into the tank to replace what you took out to clean them with. Less shock on the rock that way. I haven't had problems doing this, as long as you don't have to disturb anything and you match temp, pH, salinity...

6 months is the magic number, no matter how hard you work. But, I can't stand to look at that stuff myself.