Preheat a gas grill over moderately high heat to about 450 - 475 degrees.
Cut cauliflower florets from head, leaving behind as much stem as possible.
Working in 2 or 3 batches, pulse florets in a food processor until cauliflower is finely chopped (it should look similar to couscous). Measure out 4 cups and transfer to a large bowl.
Add shrimp to bowl along with black beans, corn, green chilies, green onions, garlic, olive oil, lime zest, lime juice, and cumin. Season with salt and pepper (about 1 tsp salt 1/4 tsp pepper) then toss gently to evenly coat.
Cut 4 sheets of 15 by 12-inch heavy duty aluminum foil. Lay sheets individually on a flat surface.
Divide shrimp mixture evenly among foil sheets, placing each mound in the center of the foil.
Bring sides of foil inward then fold together twice and to seal, then fold ends up (and work to leave a little extra space inside for heat to circulate so everything cooks evenly, I like to pull up on the top crimp).
Grill packets until shrimp has cooked through (it should appear pink in color), about 9 - 11 minutes.
Carefully open each packet and toss in cilantro. Serve warm with avocado slices.
*I recommend draining the green chilies through a fine mesh strainer. If you just try to open the can and drain you won't get much of the liquid out. Discard liquid.