Only one way really!
Cut out grout.
cut out Silicon.
clean joints and ties with bleach.
replace with anti bacterial grout and Silicon.
install better ventilation.
get a bit of heat.
leave shower door slightly open after use.
use squeegee on tiles and glass after use.
I am forever informing my customers about the importance of installing a good extractor fan in a showering environment. Some of them do, some of them don't. Going back to a job I did 6 months ago to fit one in a shower room, the moisture is running down the walls.
All you can do now is try to scrape out the grout as best you can and redo it. But be warned, the edges of your tiles will almost certainly sustain some damage.
Lithofin mildew will poss sort this , saturate in; this will remove spore ,discoloration and mould also inhibit growth. Once clean and fully dry ,impregnate grout with suitable product and this should keep any further growth on the surface easing further cleaning.