Aluminium does under go oxidation, although it is white. Unlike iron oxidation which is orange. If the tile is designed for use in a wet area it is probably coated or anodised to prevent white oxidation. Check if cleaning will be a hassle (risk stripping the coating).

Thanks for all the suggestions. I have used grouting the same colour as the tiles and finished with a smaller tile at the top allowing for a decent gap below the ceiling. I hardly notice it! Very happy with the result.

I'd replace and go with the diagonal pattern also, despite the cost (you've got to look at it for a long time yet presumably). I think the existing look comes undone because the tiles on the bench laid as they are appear to add yet another design element to the area and actually make a feature of the bench itself. I personally wouldn't want to make a statement piece of the bench itself. If the tiles were laid diagonally, I think the bench would flow seamlessly from the wall into the floor. Similarly, if you highlighted the edge of the bench with the "decorative strip" tile, I think the bench again becomes the feature piece. Lovely roomy shower enclosure though, nice job.

What a shame. Sounds like poor workmanship. It will need to be stripped out dried and rewaterproofed and tiled properly. May not have let waterproofing dry long enough or insufficient tile adhesive if tiles are moving compromising grout letting water in