Digital - sounds a lot like the bacterial build-up I get in my aquarium hoses.
If it is, an inexpensive and safe way to get rid of it
for a water cooling setup is to go to your local pet supply/fish store and get either tetracycline or myacin tablets for treatment of bacterial infections in fish. Costs under $5 and there will be enough to treat at least 30 gallons of water.
Just open the capsules in the one case or crush the tablets in the other to get a very fine powder. Dissolve it in some hot distilled water and add it to your system (after it's cooled down of course). Do this
after cleaning out your hoses (to keep it from coming back).
If nothing else, it will eliminate any possibility of it being bacterial if it comes back in a week or so
While at the store, see if they have hose brushes (small cylindrical brush on the end of a long, stiff twisted wire handle). They are great for cleaning out hoses.