I've been getting this odd brown muck on my 9 month old tank.
It is covering virtually all the surfaces and falls off the plants when I
touch it. It feels slimy.

Is it nitrifying bacteria or diatoms?

See pictures please.

http://mytriops.com/temp/whatisit.jpg
http://mytriops.com/temp/whatisit2.jpg

Tank is pretty loaded with fish unfortunately.

I had a ramshorn explosion and I took someone's advice and added snail
killer chemicals...duh....
So of course I got an ammonia spike due to the dead snails.
The ph of the tank is 6.4 @ 22C so not too lethal and I'm doing water
changes. But this damn stuff keeps coming back.