First, I would call the person who gave you those instructions and ask how he suggests getting rid of the mildew. The fact is you need to use a cleaning product on the granite to remove the mildew, and the occasional use of such products are not going to harm it. For goodness sakes, they make bridges and buildings out of granite.

If it were my bathroom, I would use Comet mildew spray to remove the mildew (rinse well after), then each time I used the shower, I would wipe the area dry with a towel. I would also improve the air circulation in the room. Find a plug-in fan that turns itself off after an hour and use it every time you shower.

Here is a link that might be useful: Comet Mildew Remover

I've used Tilex on mildew and it works fairly well.. Note the fumes are awful. avoid breathing in, work quickly with the spray and when you finish, close the bathroom door , put on the vent fan and/or open a window, My one gripe with Tilex is that it comes in different purposes-one for mildew, one for soap scum, but none combining mildew and soap scum removal.

Try getting a bottle of Concrobrium Mold Control. It is natural and approved by the EPA, contains enzymes and no harsh chemcials. It kills mold and prevents it from growing.

The only caveat is if the surface gets wet constantly then eventually the protection will wear off. Your first step should be to prevent the problem in the first place but it sounds like this corner probably doesn't get a lot of air so I would try the mold control and see what happens.

When we recently regrouted our shower, I applied a 20 year grout sealer from Home Depot. It's been 3 mos+ & zero mold or mildew. I missed one area of grout (which I'll need to throughly clean & apply sealer) because that area does have mold/mildew. The grout sealer works wonderfully!