The difference between user and my schema is

PLS-00201:identifier 'string' must be declared
Cause: You tried to reference either an undeclared variable, exception, procedure, or other item, or an item to which no privilege was granted or an item to which privilege was granted only through a role.

Action:
1) Check your spelling and declaration of the referenced name.
2) Verify that the declaration for the referenced item is placed correctly in the block structure.
3) If the referenced item is indeed declared but you don't have privileges to refer to that item, for security reasons, you will be notified only that the item is not declared.
4) If the referenced item is indeed declared and you believe that you have privileges to refer to that item, check the privileges; if the privileges were granted only via a role, then this is expected and documented behavior. Stored objects (packages, procedures, functions, triggers, views) run in the security domain of the object owner with no roles enabled except PUBLIC. Again, you will be notified only that the item was not declared.

PLS-00904: insufficient privilege to access object user.object_name

then the error would have been 'Insufficient Privileges'.

MICHEL> create procedure p is begin null; end;
  2  /

Procedure created.

MICHEL> exec p;

PL/SQL procedure successfully completed.

MICHEL> connect scott/tiger
Connected.
SCOTT> exec michel.p
BEGIN michel.p; END;

      *
ERROR at line 1:
ORA-06550: line 1, column 7:
PLS-00201: identifier 'MICHEL.P' must be declared
ORA-06550: line 1, column 7:
PL/SQL: Statement ignored