Mildew on or in silicone sealant

This is in a shower constructed with a cultured marble-like substance called Granitex. The joints are sealed with a clear silicone sealant. Some area have blackened.
I've read that mildew resistant or free silicone sealants are available, but thought I'd look into alternatives before replacing it.
TIA
Ed
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Bleach kills mold
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