It's an tumor that "once" was a pigment cell.

Melanoma in the eye is most commonly of a type called uveal melanoma. Ocular melanoma would be any melanoma in the eye, including the more common skin types (e.g. nodular, or superficial spreading, which is the most common subtype) which metastasized to the eye (very rare). Uveal melanoma confounds most of the effective melanoma treatments to date, even the most current and exciting treatments that have come on the market in the last few years. The reason (rather, one reason) is that it shares few of the same genetic targets that the more common subtypes have.

A primary tumor which metastasizes elsewhere in the body has characteristics of the primary tumor, including genetic markers, which is what allows a pathologist to determine a likely origin.