What is that brown algae called

D is for diatoms, which are not technically algae but single celled critters that often appear in new tank setups. They like silicates, phosphates and low light levels. If you are good with water changes they should subside pretty soon.
You should be able to wipe them off the leaves to some extent with a bit of sponge.

Diatoms is the name of the brown algae that often appears in new tanks, initially; it can occur subsequently under certain situations. If it easily comes off the plant leaves with your finger, this is what it is. But another "brown" looking algae is brush algae, which appears as dark brown, very dark gray, black, dark green...although technically it is a red algae. This will not easily come off.

I can't tell from the photos which this is, but your finger test will tell you. Though I would suggest it is probably diatoms, as I can see some on the glass in one of the photos. It has the appearance of a film and as I say easily comes off.

Byron.

You can wipe/scrub it off, if you want...IME it doesn't make a difference. Just tends to go away eventually, although I've had diatom spells last months.

I would try to keep plant leaves clean. Otherwise the leaf can be suffocated and the plant could die. Although I do not usually suggest fish for solutions, as they generally have other issues that can be even worse, this is a case where a species may help. Depending upon the intended stocking of the tank of course, as any fish added has to be compatible to the overall aquascape. The so-called small algae-eating fish will devour diatoms; otocinclus, farlowella cats, whiptails, Bristlenose pleco. Just be aware that these have specific needs too, and may not suit all situations.

Water changes as another member mentioned will help, along with less rather than more feeding of fish. And encouraging plant growth. In new tanks the biology is not yet established, so algae has the advantage.

Byron.