soap scum problem in dark colored slate shower

I tiled a shower with a dark-colored green/brown slate that looked great for a year or so, but has developed a white sheen to the lower half or so of the shower.

I'd like to know what product is recommended to get this off. Can I scrub the slate without damaging it?

I did enhance/seal the tile when I installed it and I will plan on re-enhancing/re-sealing it after I clean it.

www.johnbridge.com has a secial section on stone restoration. I'd ask this question over there. I think you'll get some answers.