Newbie Algy Question

[smallfish]

Hey folks im fairly new to saltwater tanks, i've been running just under a year now and my sand is started to turn brown. I thought it was over feeding but i've changed my feeding cycle to every second day, but it dosen't seem to help. My tank is 46 gal. with 20 gal. sump. 2 chromis,1 manderin,1yellow tang,2 clowns, 1 sand siffting gobbie,cleaner shrimp and lots of lps.