Grey slime in tank - what is it??

I have a small tank at my office, and the last time I did a tank
change, I noticed a grey slime in the "off-stage" portions of the
tank.
In the main tank, the environment looked normal other than the water
was somewhat dark. However, in the water pump, tubing, and filtration
areas of the tank, the system was almost clogged with a grey slime
that smelled.
I don't know what I'm dealing with (algea or fungal?), or how to treat
it.
Any help would be appreciated! Thanks! 

Tank details:
5 gallon tank with a water-fall topper (provides aeration and
filtering)
5 tiger barbs and 1 catfish (not sure of type)
No light or heat source (at this point)
Currently the PH is "off the chart", but I'm trying to get it down to
normal