Mold in a white tiled white grout bath tub surround??

Use white vinegar or tea tree oil soak in cotton balls and leave on long enough to kill the mold

Maybe this will help...

If the grout is getting loose, take it out, the mold/mildew may be under the loose areas. Wash the area well with bleach water to help get rid of the gunk and stains, follow this with a rinse of vinegar and let it dry to kill the spores. Replace the grout that you took out. When you clean out the tub, spray the areas that like to grow with vinegar to help prevent the mold/mildew from growing in those areas, especially if it is in an area that stays damp for a while. This should hold the formation in check if you continue to do this.

Go onto Pinterest. I’ve seen ideas of soaking rolled cotton in bleach and lining area until black disappears.

Vinegar! Bleach does nothing, vinegar kills the mold spores. If you have pets, be careful using tea tree oil, as it can be deadly to them.

If at all possible, improve the ventilation in the room, even a portable fan will work.

There are some very good suggestions on cleaning the grout. If it is still stained after it's cleaned, you could paint or stain it to cover the marks left by the mold. Whether you paint, stain or are able to get it looking clean, make sure you seal it to make it easier to keep clean in the future.