Silicone not sticking, mould appearing too

Renovated our bathroom about a year ago. We have marble mosaic shower base and not sure what the wall tiles are. Tried selleys wet area silicone but where the base and wall tiles meet, it is losing its stickiness. Had to reapply the silicone twice in the past year. It loses its stick after a few months. It seems to be losing its stickiness more so on the marble end. Also, mould appears, despite the selleys silicone stating that it is mould resident.

I've read that using metho helps to kill bacteria prior to applying the silicone. A few questions I have:
1. Will metho stain the marble tiles? Is metho a good agent to prepare the surface? If not what should be used?
2. Should I continue to use selleys wet area? If not what is a more suitable silicone which sticks better to marble tiles in particular?
3. How do you best prevent mould?
4. I've read that rubbing your finger along the silicone causes mould to appear down the track. Should I do this when applying? Or should a glove be worn?
5. Is spraying detergent/soap and water on the silicone necessary straight after the silicone is applied?

Any advice is appreciated.