Melt butter in large heavy skillet over medium heat. Add celery, onion, bell pepper, green onion and parsley. Saute for 4-5 minutes or until vegetables are soft but not brown. Add 1-3/4 cups water, salt, cayenne, black pepper and crawfish tails. Cool stirring often for 20 minutes. Dissolve cornstarch in remaining water and add to crawfish mixture, stirring for 10 minutes, or until mixture thickens slightly. Add cream of mushroom soup. Cool 5 more minutes over low heat. Remove from heat and let stand 5 minutes. Serve over rice.