I am not clear what exac Mac this is
or
what exact macOS you are running.
What curious is the location, and the fact it is larger than a single pixel and it is perfectly centered.
If pixel that would have to be a small cluster...
—If it is a recent app, with no icon showing, can you grab it and drag it off the Dock?
—Change user preferences to not show recent items in the Dock / toggle the option?
—Did you try Control click on it>Option Remove from Dock
—Command Tab will show all running apps on the system; Heads Up Display (HUD)
—If all else fails try a SafeBoot https://support.apple.com/en-us/HT201262
Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc.
Login and test. Reboot as normal and test as caches get rebuilt.