What is the best calking to use in the lower shower area between the walls and the base pan. Silicone does not work. It gets mold right away.

3M Marine Grade Silicone Sealant

Remains flexible and is mildew resistant.

Cure Time: Tack free: 1 hr; complete cure: 24 hrs
Cleanup: Soap and water.

West Marine sells it for around $15 for 3oz tube.

Be sure to get "tub and tile" sealant. Generic construction and caulking RTVs don't have fungicides in them. Even some of the bathroom RTVs are better than others. A few years back I compared a couple brands and some were much superior at mold resistance. Unfortunately I can't remember which were good and bad. Dow Corning was better than some, I recall.

Shop several stores and check the date. Silicone that is a little old will not set up.

I got bit by this, and research found it to be a common situation. Sadly, I did not save my research results.