Need Help

I am so pissed off with this water marks i dont know what it is called the worst part is because of this wierd shape no brush, or stick reaches at this particular curve. Do anyone know how can i clean this naturally?

I am so pissed off with this water marks i dont know what it is called the worst part is because of this wierd shape no brush, or stick reaches at this particular curve. Do anyone know how can i clean this naturally?