I had a shower with that problem and we had to get rid of it. That plastic caulk is the absolute worst. I found a product that does the same job as caulk but it's sanded and looks more like grout. We scraped all the old caulk out of the shower and replaced it with the sanded grout and that solved the problem beautifully. No cracks, no mold, no mildew. I have since used the same product in another bath and around my new sink in the kitchen and it's working great. It comes in several colors but not as many colors as grout. Home Depot or Lowe's has it. Good luck!

Just bought a used front loading washer and there were mold/mildew stains (black in color) on the rubber seal. After failing with bleach (zero difference after vigorous scrubbing with straight bleach), I used baking soda and clorox cleanup (bleach cleaning spray) to create a paste. I am sure you could just use bleach or any other cleaner with bleach but I acheived great (perfect) results with clorox cleanup. Coat the stained areas with the paste and keep it moist until you see that the stain has disappeared (took an hour or so for mine to completely fade). I had to keep adding both baking soda and bleach spray to keep the paste on these spots. As you spray with bleach the paste runs down the walls. When I noticed the paste was drying (every 10-15 mins) I would spray with bleach and then smear more baking soda. I could not find any helpful remedies online with the products in my home (i did not try any of the above methods since i did not have these items) but this was worth commenting about since it worked so well. I didnt need to scrub really other than gently rubbing with the baking soda only to apply. Success!

There is also another GE Silicone II K/B Supreme that is "mold-free product protection helps keep the cured caulk mold- and mildew-free and looking clean for 10 years, with regular cleaning." Here is a link that might be useful: GE Silicone II K/B Supreme

That Oatey sure vent is properly called an "Air Admittance Valve". The have a spring activated plate inside that is supposed to open to allow vent air to enter the drain, but close to prevent sewer gasses from escaping. Such as is common to all things mechanical, they wear out or get gummed up with residue from soap and cooking grease. The bottom end of the valve has a male pipe thread that threads into a female thread adapter on the drain line. To replace you simply unscrew and discard the defective AAV and screw a new one in its place. That is a kitchen sink line so your AAV should be 1-1/2" You can purchase a new one at any hardware store of home supply center for about $10 +/-