People often coat the battery terminal connections with grease to prevent corrosion . That could be what is attracting the dirt , etc. to collect on this particular area ? In the picture , just to the left of the grime build - up , is what appears to be a wire or maybe a vacuum hose ? There seems to be some " flakes " on that item that look like corrosion . Maybe someone ( you or a mechanic ) cleaned the battery posts and some of the corrosion was brushed off the battery and landed on that particular item ?