Black layer under decor during fishless cycling

[Nicole3713]

Hi,

I am new to this and trying to do a fishless cycle for my Aquaponics tank. It’s been three weeks and I started with RO water with minerals, bacteria, and ammonia. The ammonia levels have not changed from 4 ppm and there is a black coating underneath one of the Decour items I put in the tank and also on the rocks surrounding it. Could this have come from the decor or from not running the pump during the fishless cycling?

Any advice is appreciated