Maybe also state the specs of the Mac you're using and the macOS version it is running.
And maybe also name the effects you might have applied to the clip.
Any reason you haven't updated to FCP X 10.4.x yet?
Agree with @Gwendolini - you should update if you can.
Assuming this isn't supposed to happen (the dots are a "feature" of the template of effect) then often dots like this indicate "no information." This can happen if the rendering (sharing) bit rate is too low.
Also, does it look like this on your system, and on YouTube, or just YouTube?
On second look, I am not completely convinced this isn't a "feature." Some of the dots are white and others are purple.