Is this normal?

I've had my tank up and running for about 4/5 weeks now and after adding a hardy fish to mature the tank, I noticed that there is this odd brown build up of alge on my live sand and rocks. is this normal? I've searched some forums and most people say it's normal and part of the tank maturing. I was just looking for reassurance.

My tank is a 40 Gallon Salt with one clown in there right now. Water changes are twice a week with my tank being kept at 82 degrees.

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