I've tried scrubbing it several times with bar keepers friend soft cleanser and although it improves, it never goes away completely and it always comes back....
So it looks like ya got a couple of issues here. As the peeps mentioned, acid - weather in vinegar (preferably stonger like 30%+ if available) or chemicals to take off the lime deposits is a first start. Second it looks like the top layer(s) of the enamel are damaged and or cracked which is why you are struggling with it "coming back and not completely going away". There are fixes for this, but replacement is usually recommended if this is the case.
I'm far from a Profi in the field but I have looked into it before. There are companies that will do it either in home or take it to them, but often it is expensive. There are many home repair kits available, but the paint on / fingernail polish kinds work poorly. The sand the whole tub down and put a new layer on are better... But the cost and work typically don't justify it. If they are just top surface damages, you could try a higher polish sand job on the tub and see.
It’s likely a inky cap. They get gross and black drippy when they age. Edible when young, but never to be eaten with alcohol as they are then toxic. Best to avoid.
How do I permanently get rid of this discolouration in my bathtub? (I have hard water) ( sh.itjust.works )
I've tried scrubbing it several times with bar keepers friend soft cleanser and although it improves, it never goes away completely and it always comes back....
Spotted some shrooms on an evening walk 2 ( feddit.de )
Done with a Laowa 100mm macro