If you've tried cleaning mould from silicone previously, the mould has likely stained it. If this is the case, no amount of scrubbing or applying cleaning products will remove the stain from the surface.
Removing the silicone and replacing it with fresh caulking would be the best course of action.
To replace cracked or mouldy silicon sealant, start by removing all the old silicone using a silicone removal tool. They are inexpensive and work very well.
Once that’s scraped out, clean the area thoroughly with a mould remover or isopropyl alcohol to get rid of any residue, then let it dry completely.
If there is any stubborn residual silicone, you can remove it with a sealant remover. This breaks down the sealant and makes it easier to remove. Remember it is best to remove as much silicone as possible with the tool before breaking the rest down with the silicone remover.
Now you can apply new silicone. I'd recommend using a product like Selleys Wet Area Silicone, which is mould-resistant and designed specifically for showers and bathrooms. Use masking tape on either side of the join to keep your lines neat, then run a smooth bead of silicone and tool it off with a damp finger or silicone finishing tool before it skins over.
Here is a helpful guide to follow: How to apply silicone sealant. - MitchellMc
The old silicone must be totally removed before applying the new silicone. It is not recommended to mix old and new silicone sealers.
I suggest using the Trojan Safety Scraper With 5 Blades to strip the old silicone off. Please take extra care on surfaces like your bath to prevent scratching.
My number one tip is to make sure that the area has been washed with soap and water. Make sure that it is dry and clean before applying the new silicone sealer.
I suggest using Selleys 300g White Wet Area Waterproof Silicone Sealant. It's designed for sealing wet areas such shower screens, baths, spas, sinks and plumbing fixtures, it has a built-in fungicidal protector to stop the growth of black mould and fungus. - EricL
I had to remove all the mouldy corner silicone from a shower and used one of these sealant removers for a couple of dollars. If used correctly the corner edge is actually is very effective. The flat section is ok, and won't damage the aluminium frame, but a metal bladed scraper did the job on the flat surfaces. Removing silicone is a time consuming job at best to do it right. - Nailbag
