Your tank is cycling. It doesn't have anything to do with the dead dry stuff on the back -- it sounds like that was dry coralline.
During the cycle, you will see ammonia rise, then nitrites rising will follow. Eventualy, both of them will drop to zero, and your nitrates will rise. When ammonia and nitrites are both at zero, it means your cycle is finished, and then you can add your first animals. The cycle usually takes about 3 weeks or so.
When you add your rock, you may see a spike in ammonia and nitrites. This is totally normal, since there will inevitably have been things that died off on the rock. Just wait it out, and eventually things will go back down to zero.