I am afraid that is currently not configurable.
You’ve run into a “time window” situation.
The code in the clock applet that checks for available applications wasn’t updated correctly when Kalendar was renamed to Merkuro.
So it only picks up KOrganizer, see this comment
The check has been fixed by @carl but apparently not in the version you have installed.
What you could do is uninstall KOrganizer (since you don’t need it) and create a symlink (hardlink or copy) from the org.kde.merkuro.desktop file to its old name (or “impersonating” korganizer)