Brown algae is quite normal in a new tank. Some turbo snails will do a good job of cleaning the glass, and some good sand shifters, like starfish and small hermet crabs will keep the bottom cleaner.
I service salt water tanks as a part time job to support my hobby, And i can tell you that no matter how big or how long the tank has been set up... all salt tanks need constant attention to algae growing on the glass. so get a GOOD magnetic glass cleaner and use it often!!