The brown is zooanthelia and in simple terms is just a form of algae and can be treated as such. It can take a long time top get rid of the brown, just imagine if you had a rock with hair algae on it and couldn't use a clean up crew or manual removal, it would obviously be tough to get rid of it.
But there is hope, SPS need tonnes of light to really color up, the color pigment can be viewed as sunblock, the brighter the light, the brighter the color. Your lighting source is on the very small side and way to far away from this type of coral, so moving it up would be the first step and if you plan on pursuing SPS then an upgrade will be a necessity. You can also try a zooanthelia remover like Zeo Spur 2, when used properly it essentially wipes out all zooanthelia in the coral allowing to start over fresh. However just like other algae it will come back if the conditions aren't right.
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