Apps · Moving apps to your fast drive
Why does a moved app's Dock icon look blank for a moment?
Short answer: macOS may show a placeholder Dock icon briefly while it catches up with the moved app's new location. It fixes itself, and the app opens normally meanwhile.
If the app is pinned to your Dock, macOS can briefly show a placeholder icon after the move while it catches up with the app's new location. It sorts itself out on its own, and the app opens normally in the meantime.
That's why apps pinned to the Dock get a Caveat badge before you move them.