I would first spray the mold areas with bleach, to kill the mold. Leave on for at least 10 minutes, then rinse well and allow to dry.
Then you you could apply something like this, which is a grout pen (color: light grey in this case), to cover the discolored areas:
Grout Pen - For Restoring Tile Grout in Bathroom & Kitchen (Light Grey)
https://www.amazon.de/Grout-Pen-Rest...2s9dHJ1ZQ&th=1
It looks like that one ships here to CH pretty quickly.