How to prevent mold growth under the caulk?

The mold is growing in the morter. The only way I've found to get rid of it is to scrape the morter out past any hint if mold. Clean it with a mold killing product like bleach and let it dry on the morter. Wait @12 hours to replace the morter.

There is a caulk that claims to not mold but...

I would try not to put caulk back on it. Morter is easier to keep clean with your weekly tub cleaning.

Yes, whether or not the caulk sits on mortar, grout or in the tub itself, the mold is in the caulk and has to be cut-out, mold killed, very dry & re-caulked.

Hi Jack, Try spraying it with a tablespoon of tea tree oil in water in a spray bottle. The tea tree oil will kill the mold. Let it sit for a while before you wipe it off. If it cannot get through to the mold you will have to take the old caulk out, spray it with the tea tree oil let it sit for a while then wipe off and re-caulk. If you are getting mold though, water is getting behind your caulk so the tea tree oil should be able to get behind it too.

There are several ways to get rid of mold, but most importantly, you need to address the root cause to keep it from returning. This article focuses on the bathroom, but is an overall great article that covers methods of how to remove mold, if bleach is safe to use, and how to prevent it from coming back. Hope this helps! Hugs, Holly

https://pinkfortitude.com/mold-in-the-shower/