Combine marinade ingredients with ¼ c. water and season with salt. Add shrimp. To make relish, cut corn kernels off cobb( if using). Lightly spray skillet with cooking spray and heat over moderately high heat. Stir-fry corn for a couple of minutes. Pour corn into a bowl and add remaining relish ingredients; season with salt. Remove shrimp and reserve marinade. Lightly spray the same skillet with cooking spray and heat on high until very hot. Toss in a few shrimp at a time and cook till pink(or just opaque;1-2 minutes per side). Remove. Pour marinade into pan, bring to a boil and boil for 2 minutes; lower heat and simmer until thick. Put 4 shrimp on each tortilla; top with corn relish and marinade sauce. Sprinkle with cilantro and green onion.