Part of macOS
screencaptureuiScreenshots
○ Leave it
Takes screenshots and screen recordings.
What it is
Runs the Screenshot toolbar (⇧⌘5) and its thumbnails.
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
screencaptureuiiOSScreenCaptureAssistant
From Procession’s guide, in the words the app shows.