Lightly coat a large skillet with cooking spray. Heat skillet over medium-high; add stir-fry strips. Cook and stir until brown. Drain off fat.
In a 3 1/2- or 4-quart slow cooker stir together the water, mustard, and fajita seasoning. Add stir-fry strips, stirring to coat.
Cover and cook on low-heat setting for 6 to 7 hours or on high-heat setting for 3 to 3 1/2 hours. Using a slotted spoon, transfer meat to a cutting board. Discard cooking liquid. Use two forks to pull meat apart into shreds.
Divide meat mixture evenly among the warmed tortillas. Top each tortilla with sweet peppers and, if desired, cilantro and/or green onions. Roll up tortillas. Using a serrated knife, cut filled tortillas crosswise into bite-size pieces. If desired, secure each bite with a decorative toothpick.
To warm tortillas, preheat oven to 350°F. Stack tortillas and wrap tightly in foil. Heat for about 10 minutes or until heated through.