new tank setup

Hi

i am a beginner at marine systems and i have a question in regards to the
substrate.

my tank is still in the setup stage and has been running for a week now with
nothing in it to get the filter operating properly etc

over the last day or two i have noticed that the substrate is starting to
change from a white colour to a slightly brown colour. Is this a problem or
is this just the bacteria starting to grow in the tank which is growing on
the substrate.

Thanks in advance

Jamie