select value
from row_generator
MINUS
select customer_code
from your_table

SQL> select 51000 + level
  2  from dual
  3  connect by level <= 5;

51000+LEVEL
-----------
      51001
      51002
      51003
      51004
      51005

SQL>

select value from row_generator 
where value not in select customer_code from your_table

SELECT CUST_CD, CUST_NAME 
FROM CUST_MST
WHERE CUST_CD LIKE '52%' 
  AND CUST_CD NOT IN
                  (
                    SELECT TO_CHAR(51000 + LEVEL)
                    FROM DUAL
                    CONNECT BY LEVEL <=999
                   )
ORDER BY CUST_CD