Looks like the remnance of bird dropping or tree sap. Although that's a pretty big area for tree sap.
'11 128i M-Sport Pkg, Sport Susp., 6AT w/paddles, BMW Perf CAI,Charcoal Filter Delete, BMW Perf. Big Brake Kit, BMW Perf. Exhaust, Ferodo 2500's, Titanium shims,135 trunk lip, BMS-PBX Box-Map 3, Castrol RBF600 ,e93 FSB, , f30 Backing Plates,GC Camber Plates, Hankook RS-3's (Track setup)