An alarm in the vSphere Client UI shows that a certificate in the VMware Endpoint Certificate Store (VECS) and gives a KB link to follow.
VMware vCenter Server
This alarm is raised when there are certain number of days (customizable, but default is 30 days) remaining until the MACHINE_SSL_CERT certificate expires.
This alarm may also be triggered when there is an expired MACHINE_SSL certificate in the BACKUP_STORE.
So to clear the expired certificates from BACKUP_STORE, follow Certificate Status Alarm - Clearing BACKUP_STORES certificates in the VCSA
Ensure that you have offline snapshots of the vCenter Server, and if applicable, all linked vCenter Server nodes before replacing certificates.
Snapshot Best practices for vCenter Server Virtual Machines
VMware vCenter in Enhanced Linked Mode pre-changes snapshot (online or offline) best practice
vSphere Client UI における VECS 内の MACHINE_SSL_CERT 証明書期限切れ間近のアラーム