I started my hobby 2 months back and encountering the same symptom as yours (new-tank syndrome). For mine, it is the brown algae (diatoms) that covers my sand bed and rocks. It is very un-sight to see my white 4ft tank turns brown.
To remove those brown algae on my rocks, I find these Trochus Snail (trochus sp.) very useful. I got 3 yesterday for my 4ft and this morning, the 3 rocks where I placed them on looks 50% cleaned up!
I did water changes a few weeks back, ran bio-pellets and reducing my lights to 5 hrs per day (due to my LPS corals), the brow algae seems to slowing down.