I woulds look at iron or manganese levels in your water. This could be getting oxidized via the CL2 and leave the slime on your day tank.
There are many species of bacteria and fungi that have a ph parameter out of the range of your chlorination process. These we will call pathogenic. Similarly there are beneficial bacteria and fungi within the ph parameters of your chlorination process. You may want to take a sample of the water BEFORE hypo. Look for a species of red algae that matches your color and ph range. The only reason I can think of for not before and now in a plastic tank is the plastic of the tank is not suited to HYPO chlorine or it was coated with an oil substance which is a food source for many different species of microbes. (you are creating fatty acids that will eat away at the new plastic). The new water treatment facilities have stop using chlorine in favor of beneficial microbes. No erosion on any of facility, water is 100% pathogen free, and no possible mutational properties.
Iron and Manganese should not affect this nor should fungi or bacteria as the only thing going into this tank from new is chlorine. Pouring right from the manufacturers jug into this tank and from there it gets pumped into the water line. The only thing I can think of is a reaction between the tank material and the chlorine?