I wouldn’t worry about the brown stuff. Probably diatoms; maybe algae. Either way it’s completely typical for a new tank, and just goes away on its own.
As tanks mature, they grow all kinds of stuff. Mostly it’s harmless and goes away. Red stuff, green stuff, brown stuff. Fuzzy, hairy, whatever. If you notice a lot of brown stringy stuff with bubbles in it, read up on dinoflagellates. If everything gets coated in red gook, read up on cyano. Some cyano is normal. Dinos, less so, although a bit of something here or there that looks like dino isn’t the end of the world. .