Medical masks, as she was wearing, are not illegal. No law was broken by the lady. It was not a lawful Terry stop. The ignorance of the law by the officer, is of course, fully legal. Why should they know the laws they are charged to enforce? There’s too many of them for them to remember. It’s up to the citizens (who don’t get to use ignorance as an excuse to violate laws, as the cops do) to know everything. Or else, you guessed it; straight to jail.