What is mulm?

[reefer1]

Yesterday my skimmer stopped working. I turned off my return pump and removed the skimmer from the sump. Inside the skimmer was a light-brown, thin, film-like substance growing in small clumps here and there as well as the impeller shaft. After a thorough cleaning my skimmer started working again. I then returned the skimmer to my sump and turned on the return pump. This resulting in a bunch of this stuff being pushed into the display tank. It floated around and looked like small gel-like flakes. I believe it is mulm.
I think a thin film has built up on the inside wall of my return line. I still get strong return flow so it doesn't appear to affect the flow.
So, what is this stuff? Is it mulm?
Is it good, bad, or indifferent?
Should I clean out my return line?
Thanks!

 

[twilliard]

I don't think it is mulm

http://www.theaquariumwiki.com/Mulm

 

[Downbeach]

Are you doing any carbon dosing?

 

[reefer1]

I dose about 2ml of vodka daily into 105 gallon display + 20 gallon sump system

 

[Randy Holmes-Farley]

Skimmer insides will get coated with brown precipitated organic matter (which likely includes whole bacteria and other plankton, as well as inorganic precipitates such as calcium carbonate and silica. It is not a big deal to get some into the tank, but I'd generally try to avoid it.

Pipes can also get coated with bacteria and such. Any time my main pumps stopped and restarted, I got lots of soft white material sent to the main tank, and that's what I expected it is. I don't think it hurts anything.

 

[reefer1]

Thanks!

 

[jeremy.gosnell]

As Randy noted, this is often just build-up (both organic and in-organic). It's common, and I recommend cleaning pumps monthly, in an attempt to prevent bad build-up of this slime. As Randy said, getting some in the water column now and again, really isn't a big deal, but not something to make a habit of. The best approach is to clean the pumps thoroughly, before the build-up of such slime. I also recommend changing tubing every 2-3 months or so, to prevent an overload of this build-up within tubes. Mulm is actually a bacteria slime, known to coat the sand-bed and various filter medias. In zeolite reactors, it's very common and the reactor is actually stirred daily to release the mulm, which can act as a food source for corals. Mulm can clog the micro-pores of zeolite (and other medias) and thus must be removed to achieve maximum efficiency.

 

[reefer1]

I typically clean my skimmer and return pumps every 6 months. This sounds way too long between cleanings. Does the bacteria slime originate in the skimmer and return pump and it gets pushed into the return line where it costs the inside wall?