Depending on which verison of Moodle your on, 3.0 up included a scheduled task which synchs all users in your AD. we turned ours on yesterday. Works a dream.
https://docs.moodle.org/33/en/LDAP_authentication#Enabling_the_LDAP_users_sync_job
For older versions I think the details are here? https://docs.moodle.org/22/en/LDAP_authentication#Cron_synchronization_script