Senile Scleral Plaque
If you see a dark patch medially or laterally just anterior to the insertion of the medial or lateral rectus, this is called a Senile Scleral Plaque. It is a thinning of the sclera with visualization of the pigmented uveal tissue underneath.
A picture is worth 1000 words.
The condition is associated with advanced age and is of no significant consequence and requires no active treatment unless associated with scleromalacia and uveal prolapse. This is highly unlikely but can be treated with scleral reinforcement or a patch graft with tutoplast (pericardium) or banked sclera.