I have been battling brown algae. I have to scrub the tank often. it was covered in it as I was unable to clean it for a week. so I scrubbed the whole tank down with an algae brush (even scratched my glass a bit which im quite pissed about) pulled out all the wood since it was coated with the brown algae and wiped it down. im pretty sure now that is what caused this. but the algae is unsightly and I would love to get rid of it.