Your tank is forming brown algae from the phosphates in newly established systems. It should turn to green as phosphates are consumed.
the reason you are getting either algae is to consume ammonia, nitrates, phosphates and carbon dioxide. Just as with any plant life. And in the process plant life also filters out toxins adn even heavy metals.
to balance out and stabilize your tank, I encourage you to get a thriving plant growth, even the ugly algaes.
If you do not like the algae then establish some plant life you do like such as macros or marine plants. Protect those in a refugium if you must but get the plant life thriving.
that will make the entire system much easier to maintain, and the fish will be much healthier.