Senior cat and a brown spot on a green/yellow eye

Hi everyone,

I have a senior cat who is about 16 or 17 years old (I adopted her as an adult so I'm not entirely sure of her age, but she was likely born in 2005 or 2006).

I've noticed she has a small brown spot on her eye, which is normally green/yellow. She has odd-coloured eyes; her other eye is completely brown.

What do you think it could be? Do I need to take her to the vet or not?

Thanks.