Diatoms, excess nutrients.
It's common in new tanks and will usually stop after about 4-8 weeks. Just keep cleaning it in the meantime and eventually all will be good. Just be sure to stick to your water change routine so as to get rid of the excess nutrients as well as for all the other reasons we do water changes.
I've had my tank up for about a year now. I've got one small goldfish and three zebra danios. I did have two bigger goldfish in the tank but they are now in my container pond outside. I'll have to test the water. I haven't done that in a while. I did a partial water change last month. So I need to do another pretty soon. Is brown algae caused by over feeding? I don't think I'm overfeeding since I'm only feeding them once a day.
Phosphates can also cause brown algae. When you test your water, test for phosphates. It's possible to have a high phosphate content in your tap water.