Brown algae on the sand are in fact diatoms and the only thing that I know of that will clean them up are florida fighting conchs and tigertail seacucumbers. Neither should be added however, until ammonia and nitrites are gone and nitrates are minimal (<20 ppm). These animals do not tolerate poor water quality. Even if your tank water meets these conditions, the diatoms will disappear on their own as fast as they cam as they require mainly silica as a food source to build their shells. Silica is higher in new tanks with new sand, new plastic from equipment and the glass of the tank itself. However, if you are using RO/DI with a low TDS, silica concentrations should dissipate fairly quickly and the diatoms will fade away. Some foods contain silicates but not in concentrations to sustain a diatom bloom, IME.