buy a few otocinclus. I've had a hard time with that too. I understand it's from high iron or silica. I backed off my ferts and my plants started looking rough and the algae got worse. I bought a few oto's and they keep it well enough in check now.
Brown algae is generally diatoms which is caused by ammonia and light, have you recently disturbed the substrate or added new plants? Most of the time the filter will deal with it,give it a wipe off and do a water change and it shouldn't come back.
I've done that two or three times now but haven'e noticed much change. I got it about two or three weeks ago. I added fish recently but that didn't seem to affect it, it started before I had fish.
I think it's pretty normal to have a diatom bloom right after you cycle. I found this info page that may give you some ideas.
http/www.aquariumslife.com/headline/brown-diatom-algae-control/