I just added live plants a week ago and there has been brown stuff growing on my sand and rocks. Why is this happening, and how can I stop it?

it is most likely brown algae......how long do you light your tank for?

try you limit the amount of time you have your lights on a day, or try uping your lighting...i.e plant lights, if you are using normal lighting, the more alge you will get, use like 10,000 k bulbs and you shouldnt have much of a problem.