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[–]DarkAthena 1 point2 points  (0 children)

I'd use a bleach solution and spray it along the cracks where you're going to caulk. Let it dry completely then caulk. Do you have proper waterproofing under-layment on those walls?

If it's been getting wet under there for a long time, you've likely got rot and mold through the wood and wall board so plan to rip everything out at some point.

[–]TemporaryBoyfriend 0 points1 point  (0 children)

Mold remover is expensive 90% alcohol in a spray bottle. Get a $1.99 spray bottle, and one or two bottles from the local drug store, spray it in there, leave it for a day or two, repeat, then seal it up.

Given that you know there's mold back there, plan a bathroom reno to replace the shower within the next few years, and when you get it, replace the caulking every 2 to 3 years, even if it's "5 year" caulking.