With a new tank it will go thru a diatom alge fase.(brown dusting of algae) Thsi is normal. Just stir the sand up gently with your hand. The diatoms will go away after a few weeks once the bacteria level builds up.
Siphon out the top layer. Vacuum if necessary (usually debris may accumulate)
Place new clean sand over (if I have any unused stock) up to normal level or clean the siphoned sand in bleach (usually for several hours), wash well, rinse and put it back on top.