Anything going on in your cat's eye is enough of a reason to have the vet check it out. It may not be a cause for concern, but it is always better to be safe than sorry. Maybe you could start with sending the pic to your vet and see what they think.
The third eyelid is showing too, often related to some sort of health issue, so another reason to contact your vet.
Melenoma, a very aggressive form of cancer can show as a darkened spot on the iris. If caught early the prognossis is genneraly good but requires removing the afected eye.
My Onyx had similar spots in her eyes....they continued to grow until her eye changed from yellow to almost chocoloty - brown. I wasn't melanoma in her eye, but a precursor to it. She went every 6 months for recheck for 2 years, then to once a year. Her eye doctor said it it doesn't become cancerous within 18 months from discovery, it almost never does.
Get it checked out, if even only for your own piece of mind. You'll need a animal eye doctor, not your regular vet.
Theres melanoma and melanosis, they sound the same but are quite different. Melanosis is non cancerous pigmantation of the iris.
Melanoma is cancer, related to the skin cancer by the same name and like it, extremly agressive.