Algae question for nano reef

Hello everyone so my tank has been set up for around 3 months now, it's 12 gallons with 7 pounds of live rock and a few corals they're doing great but brown algae has recently started to grow over y live rock should I be concerned? Or is this natural? I've been have nitrate problem with the HOB filter I've ordered chaeto to make it a mini fuge is that a good idea?

Tank params. Ammonia 0 nitrite 0 ph 8.0 nitrate 5-10ppm calcium 420 I don't have a magnesium kit yet
Lighting I have kessil a160 ocean blue
Stock is black ocellaris clown
Flow is korelia 240 and HOB filter 100GPH