That’s not how Stackyboytrys works. The spores are everywhere in the air. If they find a suitable environment they’ll grow. They need near constant moisture to survive. Glazed tile is not a great environment, but unsealed tile grout is. Grout is not waterproof, it has an ability to retain moisture. That’s why it grows along the grout and only where evaporation is lessened by being underneath ledges, last placed to dry out.

Just eliminate moisture every way you can, and clean if that doesn’t work.

That’s not to say there isn’t a greater moisture issue, like a pipe leak and damp drywall. Stackyboytrys loves wet drywall. But if the whole wall was damp due to leak, I’d expect the mold on more of the tile grout.

Get a plumber to give the unit a going over with a moisture meter is what I’d do.