A worker bug

[myclan]

The worker just can't go to that tile to develop the marble

Spoiler :

 

[Little Faith]

He wouldn't be able to develop the marble anyway as it's outside your culture borders.

That said there is some kind of limitation as to how far outside your territory worker units are able to move. I have yet to figure out the exact mechanics though.

 

[Nightstar]

It's because there's an unexplored tile 2 tiles SE of the marble and Workers can't reveal unexplored tiles. Workboats have the same limitation.