Looks like your tank has the same problem as LittleWatty maybe? They provided much more disgusting pictures then you did though. ;) I think LittleWatty wins the most disgusting aquarium problem picture award with their second picture ;) (In my book at least lol)
Some of the brown stuff might be diatoms too. If it smells, it sounds like you probably have more then diatoms though. (....bacteria festival lol..... if your tank starts to give away T-shirts to commemorate the festival in the years to come mark me down for one lol
Hope you can get rid of whatever it is though.
Edited to add:
Sena in the post I linked says what the heater gunk smells like even though she never did explain what happened with her heater to cause slime.