For immediate removal, scrub the affected grout with a solution of one part distilled white vinegar to one part water. Avoid bleach; it only removes surface discoloration but doesn’t kill the root structure and can weaken grout. For porous surfaces, this may only be a temporary fix.

Preventing regrowth is about controlling humidity, as no sealant is a permanent solution if the underlying moisture problem remains. Since your exhaust fan isn’t sufficient, verify it’s actually moving enough air by holding a single square of toilet paper to it; it should be held firmly in place. Run it for at least 30 minutes after every shower. Wiping down wet tiles and the shower area with a squeegee immediately after use is the single most effective habit to reduce standing moisture.

In a windowless bathroom, a small desiccant dehumidifier can make a significant difference. For the grout, once it is completely clean and dry, a high-quality penetrating grout sealer (look for epoxy or silicone-based types) can help. However, if the mold is widespread, deeply rooted, or you suspect it’s behind the tiles, a professional inspection is wise to assess the full extent. They can use moisture meters and borescopes to check for hidden problems that simple cleaning won’t solve.