Freezing will eventually cause the bladder to fail way faster than if you don't freeze it. If you put about 1 TBSP of bleach in the bladder and let it sit, full, for 1/2 hour, and then bleed it out through the tube, it will kill anything that might even be thinking of growing, though. After running the bleach through, re-fill with water, bleed it out, and air dry. Unless there's something funky in your water, it shouldn't grow mold. If you're on city water, and mold is growing, you ought to call your water dept. & find out exactly what the hell they are doing! If it's growing in a clean, dry bladder, imagine what's going on in you household pipes!
If you have well water, you might pour a gallon of bleach into your well (at least 3 times a year), then run your water until you smell the bleach. As soon as you can smell it, turn off the water, and let everything sit for a few hours. Them, turn ALL your water faucets on and run the water until you don't smell bleach anymore.
PS - if your refridgerator dispenses water/ice, turn the icemaker off, and remove the filer, or the chlorine will kill the sucker......