Is this algae or something serious that needs treating?

[sharkfur]

Hi everyone,
I spotted some round patches of dark (dark brown, blackish?) on the usual green algae I let grow on the back surface of my fish tank (fresh water, only platies and some plants). So, I don't clean the back surface of the tank off of the green algae, and I would like to show you what started growing on it. Please tell me it's just brown algae and not some serious fungi or rotting thing.. Have a look at the photo please, and let me know what you think.
(You will see some grungy texture, but don't be mistaken, that's just the actual wall behind the tank)
Thank you very much in advance!

 

[mbsqw1d]

Yeh brown algae (diatoms). Is this a fairly new setup?

 

[sharkfur]

Yeh brown algae (diatoms). Is this a fairly new setup?

Oh, that's a bit better than what I imagined. I never seen brown algae appear in little circles like that before.. Uhm, no, it's not a new tank, roughly 5 years old. Thank you so much for replying