I wouldn't get too excited about brown algae in a new tank like yours. Its just diatoms and is paving the way for the more desirable coraline algae. You will have an algae bloom that is worse than the diatoms, just wait. When its all finished cycling, it will come out alright in the end. Use good quality water (RO/DI), good salt, good lights as eleyan said and make sure you have some clean up critters, such as snails, in that tank. Start collecting them now, cause otherwise the next algae bloom will really challenge you and you'll be way behind on collecting the critters.