Depending on your flooring cover it may be due to age or wear, also it could be due to constantly being wet And damp
it sounds like damp,mould built up due to lack of ventilation or a leak.I would have checked to make sure you have an extraction fan to take away the excess steam from the shower.also check to make sure there are no leaks around the the shower
If it is on a concrete floor there is damp underneath. Either water is getting under somewhere or coming up through concrete.
Steam clean, use a good cleaning product.
This is mould build up, water must be seeping through somewhere and building up
Sounds like mould and damp, you may have a leak somewhere
This is mold being builded up cause of damp situation. You can clean it with vinegar and wash it away with hot water, also if there is lack of fan than do install,leave your bathroomdoor/window open after taking shower to avoid such situations in future. .
Hi, shampoo and conditioner can react with the grout, it needs taking out and swapping with an oil based epoxy grout it's much easier to maintain.
It sound like you haven’t got sufficient extraction in the bathroom causing mound to build up
It might need some chemicals to clean it depends when you say shower floor gone black