I think you are talking about the hyper pigmentation in the white part of the eye( sclera/ conjunctiva
) rather than the one on the cornea
Because, Cornea is the transparent part of the eye and any hyper pigmentation in it is called a scar/ opacity which will affect your vision too.
So I am answering this with an assumption that it is either conjunctival or scleral hyper pigmentation .
Since you got it just 5 yrs before ,it is an acquired melanosis
which is progressing too( darkening).
First get it examined by an eye doctor to see whether it's just a mole ( naevus) or melanoma
If its a conjunctival mole, it can be removed by simple excision alone.
If its a melanoma , A biopsy
of the lesion is usually performed to find out whether it's benign or malignant before excision.
So get in to conclusion of your problem ,after slit lamp microscope examination and necessary investigations at the earliest. Take care.
Do let me know whether it's corneal or conjunctival!....:).