Airline tubing is made of silicone or sometimes vinyl, it's not the same thing. Is this tube flexible? Are you sure it's PVC? Milky white + slimy sounds like fungus to me. Have you tried scraping any of it off? Can you?
What are your readings for ammonia, nitrite, nitrate? How long has your tank been set up? Did you wash the tube before putting it in, and if yes, how? How long did you have the tube in the tank before it turned white? How long after that before things started dying, or did they before the tube turned white? Were things dying before you put the tube in the tank? Is anything else in the tank growing the same white stuff?
I placed it on my aquarium around the same time as Black Friday November 25 2016 and today being 12/11/2016. I also added fish to my aquarium which I assumed was the cause of this massive fish death. Some of the dead fish I took out were covered in the same kind of milky white covering. Its looked like cured superglue.
My readings are all normal, tank has been setup over 1 year, I did wash the tube before placing it in the aquarium but I also used aquarium safe Silicone which I let cure for over 2 days, things were NOT dying before the tube was placed in it( but as I said, I also placed new fish in the tank at the same time) the tube was in the tank for maybe a week before it turned white. Maybe sooner, I'm not exactly sure.
Stuff started dying the next day after I got the new fish/placed the tube in the aquarium. I have probably had to remove 25 fish so far.
I place replaced it with the original outlet and did an emergency water change of 40 gallons(in my 75 gallon tank) followed by another 40 gallon water change a few hours later. Hopefully to dilute any type of chemical that may be in the tank.
I also ordered a powerful 55 watt UV sterilizer which should be in on December 15th in addition to a 9 Watt internal UV sterilizer plus another 9 Watt UV sterilizer in my canister filter.
I have a 500 GPH canister filter as well as a 264 GPH secondary canister filter. There does seem to be a white milky pasty slime substance which I can see when I run my finger along it. The tube still stays the milky color. It does not turn clear when I run my finger along it.