During growth of the dilute p-type ferromagnetic semiconductor Ga1-xMnxAs, interstitial manganese, Mni2+, is formed when x exceeds 2%. The double donor Mni2+ compensates the free holes that mediate ferromagnetism. Annealing causes out-diffusion of these interstitials, thereby increasing the Curie temperature. Here, we use cross sectional scanning tunneling microscopy and spectroscopy to visualize the potential landscape which arises due to the clustering of Mni2+ in annealed p-i-n (GaMn)As-GaAs double barrier heterostructures. We map the local minima in the potential landscape, link them to clusters of individual Mni2+ ions, and show that the ions are doubly charged.