You should vacuum it out, have you done regular water changes? and test for ammonia, nitrites, nitrates? Also, in my opinion 15-20 lbs. of live rock and 5 lbs. of live sand won't do for a 150 gal
tank, there is a chart in this forum that will help figure out how much stuff you need. I will also help if you have a lot of water circulation, do you have a sump? or powerheads in your tank? Brown algae is easy to clean up, although it comes back quick, which is why you need to do H2O changes and have a good filtration system, when the algae turns green then it's hard to scratch off. Let's see what the others have to say.