Quickly Identify Invalid Choices

A suggestion would be to use the dynamic filter and sequentially check all of the drop-down choices one by one. When do you find a block of red ones,  clean them up, and then continue working down the list of choices until you don't find any dynamic filters choices where the the records are in red.

The other option would be to create a table for the valid multiple choices where the key field of the table is the choice text itself. Then make a relationship back to your detail records based on your existing multiple-choice field.

Then to create a field called [Choice is valid?] valid with a formula checkbox field of true on the Choices table and look that up down to the details table. Then where ever the [Choice is valid?] is false not exist you can trap those in a single dynamic filter.  

I typically try to avoid using multiple choice fields except for obvious things like approved / not approved, yes/no etc.  If it's going to be from any kind of long list at all, then I prefer to make a relationship where the valid choices could be properly controlled by authorized users.  I make the key field be the text value of the choice.