My best guess is diatoms or brown algae
I have read that diatoms usually occurs in newly setup tanks AND/OR your water have high amounts of silicate. Also, it seems that low light is another cause as well.
In my opinion, don't get too frustrated with this algae because its really not that bad. its not bba or cladophera (those things are dreadful) so don't stress out too much. I have diatoms in my 29g for 2 years and its still there hahhahha.
If you want to get rid of it, get some otocinclus, snails, or bristlenose plecos. they should help reduce diatoms. Also, increase your lighting. That should really get rid of it. I had diatoms in my 75g until I replaced my old fixtures with two finnex fixtures (48" planted+ and normal fugeray). It increased the PAR to high-medium (I think) and I have not seen any diatoms since.