Cleaning bathroom tiles reveals gray lines. What's happening?

Question

I recently moved to a new apartment. The bathroom tiles were gleaming white. However, the first time I cleaned them, suddenly darker gray streaks appeared in the tile -- in random spiderweb patterns, looking a lot like cracks that had been somehow filled in. Scrubbing with a powder cleanser like Comet revealed the lines slowly, but a splash of rubbing alcohol revealed them instantly.

Are these repaired cracks? Did my landlord do something to the tiles to hide these flaws? Is this worth complaining about?

The grey lines are ugly. Can I get them back to that white state and still clean them?

Answer 1

Call a professional tile and grout cleaner.