The way you are trying to do this isn't possible. Salesforce docs:
Field to field comparison in WHERE clause of SOQL
Description
I want to execute a query by comparing two fields of the same object in WHERE clause, but I am not able to use a field in
the right hand side of a condition in a WHERE clause:
List<user> users = [SELECT Id,name FROM User WHERE (FirstName != Lastname)];
The above query returns: "System.QueryException: unexpected token:
'Lastname' "
Resolution
Salesforce doesn't allow direct field to field comparison in SOQL query.
To achieve this you may create a formula field that will compare
fields and return a value (like true or false) which you may use in a
WHERE clause.
So for the above query, you could create a formula field on User
object with return type Text e.g. NameCompare, with the formula
IF(User.FirstName != User.LastName, 'true', 'false')
Now our query will be:
List<User> Users = [SELECT id, name FROM User where NameCompare= 'true'];
Following idea has been posted on ideaexchange portal for allowing
field-to-field comparison in SOQL:
https://success.salesforce.com/ideaView?id=08730000000BrHAAA0
In your specific scenario, you will want to create a formula field of type Checkbox like:
LastModifiedDate > CreatedDate
Then rewrite your query with your WHERE clause like:
SELECT count() FROM Case where Last_Modified_Greater_Than_Created__c = true