It looks a bit like red algae which I had problem with a while ago but no only grows on the outflow of of my hob filter. I hear that it is encouraged by excess organic sin the water column so try doing more water changes and removing as much of it as you can. I also found that increasing lighting in the red spectrum helped becouse plants are more capable of using red light than red algae is.
I would clean the whole tank, knock every thing off the walls and decorations. Then immediately change the the water. Take all of the bad plants and wash them in a 20:1 bleach mixture, if they don't live through that they will not live, afterwards dip them in water heavily concentrated in prime. Put the tank back together and try not to let it get that bed ever again. As for the cause, how long are the lights on for and how do you measure your co2? Remember trying to measure co2 by bps is like putting you hand out the window of a car to see how fast it is going. Just trying to help, we'll get to the bottom of this
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.
Yup, looks just like the stuff I used to have (though mine was more red than purple ). If you have a microscope handy you could look at it under that to see if it's an algae or a bacteria. I had asumed that it was a kind of red algae like BBA but I'm not sure of that. Viewing it under a microscope would (dis)prove this theory.