Hm i havnt noticed any white slime on my airstone. My setup is below my 29gal tank but instead of using 2, 2L soda bottles im using just 1 for now to see how it does with only 1 if i need 2 i will be adding the second. Are you using check valves? (not sure if they help in your situation) since im still new with the whole DIY setup my self. Right now im getting plenty of fine bubbles and am quite please with the co2 output but will most likely switch to hagen ladder. Keep us updated on how it goes
I have this same gunk on my limewood airstone. The bottle is prob 3 ft below the tank and i have a DIY gas seperator level with the tank. The gunk remains and has done so since i can remember using limewood. What I do is get 2 airstones and then switch them out with each batch. After i switch a used one out i soak it in a mild bleach solution for a day or two and then put it aside. Some of them can be used for more than one batch while ive had others that just wouldn't work right.
I think the gunk is the limewood breaking down. When CO2 dissolves it turns acidic and I believe that helps break the wood down. That or bacteria are digesting the wood. If I had a microscope at home id actually like to see if the gunk is teaming with bacteria... could also be fungus.