No need to overthink it. If its a single mite on a single worker chances are its simply hitching a ride or may even be there by chance, simply seeing the ant as another surface to forage on. If the mite appears to cling on the ant without moving, then it may be a problem, however chances are likely its just phoretic. Best to take a sample and try to ID it before panicking, mites arent the demonic killers the pet hobby makes them out to be. More often than no they're harmless or beneficial.
I agree with this, most are harmless, unlike the way some people make them out to be...*COUGH* ANTSCANADA *COUGH*
Submit a off-topic ID request I guess, and see what people say. I have heard lemon works to get rid of kites though if it becomes a problem.