Instead of a FTS, can you get a close up of the algae itself? What have you done about it so far, and what has been the algae's response? Have you tried to scape it off, and if so, how easy/hard is it to scrape off and how quickly does it return? How did the algae emerge; quickly or gradually?
This doesn't look like any kind of algae I've ever seen. If it was green, that would be different, but it looks very dark and you said it's a brown color. I'd suggest diatoms which are usually what gives the brown algae appearance, but that doesn't look like diatoms from what I can see.
Have you noticed any responses to it with your fish, inverts or snails? How are your fish doing? Everything healthy? Any recent deaths or illnesses? What are your water readings for ammonia, nitrite and nitrate?
A close up picture and more detailed info would be very helpful.