I am a newbie, but from what I have read on this forum, the brown algae you are experiencing is a normal cycling condition, perhaps even a favorable one.
My tank cycled, and experienced a bounty of brown algae growth all over my beautiful white sand. Provided your water is clear, and your test levels are normal, I think you might be okay.
I am sure some of the others here may be able to advise you better, but you may be ready for a clean-up crew to munch on the brown algae.
Can't really comment on the one affected fish.