Sealant turning black from the inside

[pooksahib]

Hi. I've read a few threads about mouldy sealant but none mentions my particular issue - turning black from the inside out. It's a fairly new build I'm in and the original sealant looked like this:
View media item 64057
I replaced it last summer and this is it today:
View media item 74900
You'll note the greyish bits as well as the more developed black areas. And that's despite always squeegeeing the walls and wiping the shower rim after every use. The blackness doesn't seem to be tangible which is why I think it may be developing 'inside to out'.

Is that possible - moisture getting in behind the tiles maybe?

Any suggestions gratefully received.

 

[foxhole]

Not clean properly before original sealant went on, needs to be spotless to adhere and prevent mold getting in, ventilation keeps everything dry prevents mold spore settling and growing.