I’m simply trying to mirror a subtool but for some reason (if you look closely) the mirror has infinite tiny dots going vertically down from it 
What on earth is going on?!
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clear your mask.
how are you mirroring your mesh?
Deformation > mirror > x
I found one solution is to make a selection (green box) around the piece and then geo>modify topo> del hidden
instead of deformation you should try 1 of 2 things.
If you have subD levels and want to keep them you should use subtoolmaster>mirror
If you don’t have subD levels you should use geometry>modify topology>mirror and weld.
That will save you the step of having to delete the geo, or having any stray ones that you might miss or not see.
Ah I was wondering about that, yes those other options sound much better.