Part of macOS
calaccessdCalendars and contacts
○ Leave it
Keeps calendars, reminders and contacts in sync.
What it is
Syncs your calendars, reminders and contacts with iCloud and other accounts.
If you stop it
macOS starts it again straight away. Leave it.
The verdict
○ Leave it. Part of macOS, or something that starts again by itself. You never need to close these.
In Activity Monitor
calaccessdreminddcontactsddataaccessdexchangesyncd
From Procession’s guide, in the words the app shows.