Both my tanks did this when they were new. I didn't do anything to stop it,it stopped itself. I did get tired of looking at brown substrate,so I just stirred it up a bit to bring up the white. Steve
Sometimes adjusting the current in your tank will help. Not enough or too much will help the algea out. A cleaning crew will help for sure. Sand sifting gobies are real fun to watch and do a great job at keeping sand white as rice.
I hope your tank is mature,with lots of live rock crawling with bugs. The manderine may eat some flake food,but their main food is the bugs found only in a mature tank. STEVE