infinite zest I've lived in places with much grosser bathtubs, believe me. That looks like black mold, something that occurred at a place I lived a couple of years ago. As much as I tried the bathtub always looked about like this one, and that's because of constant turnover.
Fun story, I got one of those NASA beds or whatever from my neighbors when I lived there because they had to head back to New Jersey, and I was so excited! Those cost like a thousand dollars! But I got it into my apartment and stated wheezing like crazy. Turned the thing over and it was covered in all sorts of mold. How they slept on that thing.. I dunno dude.
But glad to know you keep a tidy bathroom in case we ever have to be roommates some day. :)