Dirty tank

I'll guess and say that your tank is fairly new. If so, that brown stuff is diatoms and common to new tanks. It will go away after a while. It might take longer if your water is full of silicates. I recommend water changes with RO/DI water. That pretty much eliminates the silicates and that brown dust.

Good advice from Axepilot...
In a fish-only system this will gradually shift to green micros; which is not a bad thing at all. If you're planning a reef system it's a different story; brown diatoms/red cyanos/hair, etc. can all pose a more serious problem for corals and LR.

Water movement and clean up crews

Make sure you are turning you tank( hence currents in the tank) if you are allowing the currents to eddy at the bottom( if you can't see your sand moving at the bottom occasionally)..increase you water movement....more towards full movement( surge)...not just direct stream( powerheads) as this will cause stress. With out movement you will develope a nutrient sink for the daitom algae/hair algae...soon it will turn into red slime(which is not even an algae as more a bacteria) and that is in my opinion worst than hair algae. get youself a good selection of janitors....Snails.....they do the most cleaning, stay away from "Hares" they are a "sea slug" they will die very rapidly once the source of food is depleted, and one adult Sea hare can deplete a 200 gal of hair/daitom in two days. Hermits will eat and damage what little coraline you have got( not to mention they will also eat your snails if they get the taste of one.)

15G Micro
175 12K Sunburst MH
96 Actinic 3 PC
Prism Protien
RS Wavemaster Pro
Rio PH750, PH 400 and Rio 90
30 ocean caught snails( looks like astrae)
Grey Abalone( 9cm)
Leather Colt Frag. 4"
Xenia
Green bulb Anemone
Lobophysilis multiColor
misc Mushrooms
Moonbrain(Closed)
Dusters
1 2 yro ocellaris, 1 5m juve
Reef Cyrstals, no addition additives
10% WC weekly