Toilet replacement or just mold?

[Masa Asao]

I have a renter asking me to replace their toilet because the toilet's inside rim has "black" stuff that they can't get rid of. Even if they get some of it off, it returns in 2-3 days.

We're in the northwest where it's rainy and damp all year long and as far as I can tell from investigating the internet and youtube (yes, the source of all truths lol), these are mold and yes, they are hard to come off. They sent me pictures and video of the toilet's inside rim.

Does anyone have similar experiences? Are these mold or should I really replace the toilet?

NOTE: Please be warned that the images contain pictures/videos of inside a toilet!!!

https://1drv.ms/u/s!AsQMTIUZ_mlll4EJzxBDLWfwazLDQw?e=nzFaXA