Black Rock Question

Well this question could go here or sand forum, but live rock is the main part of the questions, so I pickes this forum, he he.

I was redoing my aquascaping today, and found that the part of the rock that was under the sand had turned BLACK!! Oh and stunk really bad.
I dont have a deep sand bed, it is only 2 inches, with some spots around 3. The sand is super fine , with two bags of special grade live sand. 1-2 mm. The tank has only been running for 2 weeks, so im not to sure what is going on. Is my tank rotting, or is this normal. My other tank has crushed coral, so I never got this problem. I read that the finer the sand the less you have to have for the deep sand bed affect, so mybe the bottom of the tank is not getting enough oxygen?..... Hopfully someone will be able to help me with this,

Michael

 

Can you post a picture? I don't think it is anything to worry about. Probably just some sort of growth...maybe algae. Hard to tell without being able to see it.

 

ill have to get a pic tomarrow, but this stuff was on every rock that was under the sand. Im guessing it is part of the cycle, but not sure.