Did you contact the company you purchased the product from for more information?

Personally, I would clean it with a hand-held steam cleaner too - but I'm not sure if your garment steamer has enough pressure to do the job like a small steam cleaner has, but certainly worth a try since you already have one.

I'd also make sure to leave the bathroom vent on for at least 20-30 minutes after anyone takes a shower, which is going to help draw out the moisture in the room. Perhaps a higher grade of bathroom vent is in order. If that isn't in the "picture", you may benefit from taking shorter showers to reduce the amount of moisture in the room. Perhaps a well-placed fan to dry the air would help.

-Grainlady

Tile pro or Tile Lab makes a mildew remover specifically for stone/marble.
As for soap buildup, I use pine sol diluted to clean my tumbled marble shower walls