OP is not coming back, having been overcome by some horrible disease associated with squalid conditions.
That'll learn her not to use the bath when she's only been invited round for coffee in the first place.
In all seriousness, it looks like someone has cleaned the bath, and then a bit of dist has settled in it post cleaning, or maybe they rinsed the cleaning cloth out in it. If that is the dirtiest part of your host's home, then they are very tidy people.