I use Flash Gel with bleach for the purpose. Quite effective.
But read the instructions to make sure that it will not damage the surface of the bath. But if you apply it carefully just on the sealant, should not be a problem.
best to take off an reapply with mould resistant silicone ,make sure you fill the bath full when you reapply , you can get a handy lil silicone tool that gives a very nice finish
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