Is it a powdery type of brownish gold algae?

If so, its diatoms. The fact that it is on the substrate makes me think that is probably what it is. Diatoms are extremely common in new systems. Simply just part of the sycling process. The leading cause of diatoms is excess silicate as well as phosphates. After your systems matures you can elininate them with some snails. Astrea, Turbo, Strombus, and Trochus snails are herbivores of diatoms. Wait until your system settles down before trying to add any snails. They are sensitive to ammonia, nitrite, and nitrates.

You may want to try to siphon some of the diatoms off the substrate with some small diameter tubing in the mean time when you are doing water changes. You can fray the end of the tubing with a small pair of scissors to make a "brushy" end to help scrub it off the LR.