Part of macOS
UniversalControlUniversal Control
○ Leave it
Shares your keyboard and mouse with a nearby iPad or Mac.
What it is
Runs Universal Control and Sidecar.
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
UniversalControlSidecarRelay
From Procession’s guide, in the words the app shows.