Preheat grill. In a bowl, whisk together olive oil, Dijon mustard, juice from lemon wedge, garlic and parsley; add shrimp. Chop green pepper and onion into kebab-size chunks. On wooden skewers, alternate shrimp with cherry tomatoes, onion and pepper chunks, and mushrooms. Grill over foil, flipping periodically, until shrimp turn pink on both sides, 6 to 8 minutes. Serve with corn on cob, sprinkled with garlic powder.