Are you cleaning with bleach?

You need to clean mold with vinegar, not bleach. Vinegar will keep mold from growing back. Bleach will not.

Sounds like black mold this could be a couple of things, Either the tile is pourous or the substrate was not waterproofed properly and is holding water in that area.

Try using an oil of cloves solution - put 1/4 tsp oil of cloves and a litre (about 4 cups) of water in a spray bottle, spray on and leave for 24 hours, then clean with a mixture of baking powder and vinegar. Make sure the oil of cloves is properly diluted.

I am cleaning with hydrogen peroxide

Learn something new every day. Vinegar instead of bleach. Thanks Bev.

You are very welcome Angel! I learned this on something I was watching on PBS regarding keeping mold away.

We just had a handyman replace the silicone sealant. What was used originally apparently wasn't the type for bathrooms and wasn't as water resistant. They scraped out all the old stuff in the corners where there was mould and replaced it. Works so far!

I would have a grout pro. remove it, confirm there is no hidden mold, and repair it.

Bev - just used vinegar on a couple of windows. Worked like a charm in removing it and didn't ruin anything as bleach often does (my clothes/hands).

I did not know that about vinegar either. Good to know! I did just read something about using vinegar instead of Roundup to eliminate weeds (as long as there are no other plants around as it will kill them also). We always have issues with weeds coming up between pavers on our front path and usually resort to using Roundup because it does work and keeps it weed-free for most of the season, but this year I plan to try vinegar, the miracle liquid!