Probably nothing to worry about. I got the same thing a few weeks ago (I'm still a newbie as well). It seems it is normal that first brown algae appear. I was worried as well (posted a thread here
and indeed some replied with my current answer).
At the time I did cut back on the hours lighting (from 12 back to 8 hrs). After a week all the brown stuff was gone. I am currently back to 8 hrs whites, 10 hrs blue. Rights now I'm facing a small amount of green algae (not hairy or bubble). This also seems part of the cycling. What I start to learn is that "cycling" is not JUST ammonium & nitrite back to zero. It is a lot more, including algae growth.
Good luck!