Welcome aboard! What you are experiencing is one of the challenges of reefkeeping -- algae control. It is normal during tank cycling for brown (and green) diatoms to appear. Over time, if you keep your water parameters in good shape (8.1-8.3 Ph, 8-12dkh Alkalinity, 400-450 Calcium, 0 (or near) Ammonia, Nitrite, Nitrate, Phosphate) you will find that the coralline algae will start to return. You will probably also notice a number of other macroalgae species starting to grow. Just be patient and keep on top of your maintenance.