brown stuff on glass

[fishguyxd]

I have had some rustic brown film coating my tank glass, at the moment I can't remember what it is I believe its a algea, over the past few weeks I had low ph & carbonates in the tank and have since only added reef builder,but this film grows fairly rapid not covering the whole tank but blochy in only a few days.Anyone give me a quick review on what they think it is and what I should do thanks I have I believe 90+ lbs of LR and only 5 fish ,most small so not a heavy bio.