Help, rusty brown stuff growing in my tank!

A month ago, I thought one of my plants had been burned by too much
light. It was dark brown/black on the top of the ferns. It had the
problem with no ill effects for over a month, but I decided to moved
it to a corner of the aquarium hoping it would do better, fool me. At
the same time, I bought two small eels and have been feeding more
blood worms than usual. Nothing else has changed. Now, I have this
rust coloured stuff all over the place and all of my other plants are
dying. I pulled the original plant out in a desperate attempt to stop
the flow, but to no avail. Any ideas on what I can do? My farlowella
and ottos seem uninterested in it and it is now covering decorations,
rocks, bog wood….you get picture. The aquarium is 50 gallons and I do
a 25% water change each week. I have noticed there is more gunge in
the filters lately, but I assume that is this brown guck.