Brown algae commonly appears in newly established saltwater aquariums. It is not true algae, but rather an accumulation of single-celled organisms known as diatoms. Diatoms appear in response to silicate, which occurs in new water added to an aquarium before filtration is established to remove it.
The easiest way to get rid of diatoms is to wait until the aquarium is more established. Once the filtration system is mature and contains beneficial bacteria, the silicate in the water is consumed and filtered out. This leaves nothing for the diatoms to feed on, and they starve to death. When brown algae appears in an established tank, silicates in the tap water may be an issue requiring chemical treatment.