Brown layer on top of my water

[Trevor Robertson]

I am in week two of my cycle, I have a 135 galon tank and 100lbs of live rock in it to help the cycle move along. I have noticed that a brown layer of dirt? has formed on the top of the tank, I have not seen this before and if I wipe it off it comes back in a matter of minutes (well half our but still quickly!). I have talked about this at my local LFS and they said that it is normal but could not tell me why or what is causing this, could you help.

Thanks