The bathtub caulk is moldy.

You've got to remove every bit of old grout so that the new material forms a seal against the tub and tile. If moisture gets back there, mold will form again. Use a 5-in-1 tool and a plastic razor to cut it out, and caulk remover to loosen any remaining bits. Then apply new acrylic latex caulk containing mildewcides.
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